You go talk to your daughter about it, that's what you do. Even if she won't listen. (Having been the rebellious teenager, and one that obsessively spent all her free time on IRC...) Why?
Because you should teach her what to do and what not to do on Myspace, instead of blocking it. You can block iChat/Adium/Proteus/Fire/AIM/Mercury Messenger/aMSN.... all you want, even aim express and meebo, and she'll still be able to figure out a way to get past that, especially considering how you could probably do a google search for such chat clients, write your own, things like that. It's not just Java. Meebo's using ajax - no java. She can use proxies to do whatever she wants. Google cache to view websites you blocked. I mean the list goes on. There are a billion ways to get around these limitations.
It does not take a genius to realize admin passwords can be reset with an OS X disk, or that blocked websites can be viewed in other ways.
So give up on the blocking and
show your daughter how to act appropriately. Also, don't be like a lot of parents and start screaming your head off and automatically write off everyone online as being a sexual predator. The internet when used appropriately can be a very, very useful tool.
If somehow you think she'll do what you want after you block her off like that, you're very wrong. Nothing stops her from going on AIM/myspace from school or at a friend's house, or random wifi networks in the area.
Also, don't be too harsh. My parents were annoyed I spent so much time on the computer when I was littler (like, your daughter's age), but I'm a CS major now in college, spectacular grades, get complimented by everyone for my work and I enjoy what I'm doing right now. And surely enough, spending all that time doing seemingly useless things was part of the reason why
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(the seemingly useless things included lots of slashdot reading and such though...)
And on a final note about homework never needing a computer: not true. Maybe 5, 10 years ago that was the case, but nowadays one needs access to a computer, especially the higher up you go. In my case, I know I've been required to have access to one since maybe, 6th grade english class like 5 years ago. You just NEED to get things typed, seriously. Last HS english classes I took forced us to submit our papers online to turnitin.com... Papers needed to be typed, and only in rare cases handwritten papers were allowed (usually because this meant the person couldn't procrastinate and write the paper an hour before class...also because the paper could be checked online for plagiarism, which is all the rage now...if it was handwritten we were also required to submit it online, otherwise major points off..)
Tis also the case in my college math and science classes - a lot of our stuff is done on webassign/webct/blackboard whatever. Needless to say my CS classes sometimes require visual studio .net as well.
Edit: Facebook is actually miles better than Myspace et al considering how you NEED to have a working college email address to join (not talking about the HS one, even though they sorta work together now), and the security limitations on Facebook force you to consider who you display info to, and how much of it to show so it's less of a freeforall compared to places like myspace and xanga and lj. But what comes first is knowing what to say and what not to say, and that's a valuable skill that can be used everywhere.
Second edit: Blocking ports is useless, it's extremely easy to change in apps that are dependent on port numbers (bittorrent and p2p stuff), and usually all data from aim express and websites like that get routed through port 80, which happens to be the port for http stuff, which if you block, you can't use a large chunk of the internet. Also, some sites depend on non-standard ports, or the alternate port 8080 for traffic, sometimes legitimately, so going around doing that isn't great.
Jeez, third edit: The best is to have a good relationship with your daughter regardless of how she acts. Distancing yourself from her by placing such restrictions results in a more distant relationship which is disastrous if she does something she would have normally told you about but didn't because she was pissed off at you, and that something has severe consequences. Also, it tends to be less stressful and more enjoyable when everyone communicates with each other. I've noticed this dramatically late last year, my parents and my relationship became a friendship (in the good way) and less of a annoying parent/rebellious pissed off teenager thing going on, and all the stuff involved becomes better for everyone (more freedoms, less worrying, more open about things). Treating her thusly will only lead to more rebellion.
I really wish I could delve more deeply into developmental psychology, but alas I'm only in intro psych. but the one most valuable thing I've learned is that the (oft cultural/social) perception of a infant/child/teen/adult mind tends to be different from what's actually going on, so don't go yellin when it's inappropriate (i.e. punishing a child for not being able to control him/herself...considering how the area of the brain responsible for impulse control doesn't fully develop until the late teens..)