Can I defend my mac against this?

[hassel]

last night my nefew came over and was on the imac for a couple of hours, when he left I went on it and found it in a mess.
flash player was trying to install itself and was stuck at 40%
mac cleaner pro was installed and I couldnt close it down

in the end I had to do a clean install of mojave. lol

how do these programs install without user password, so my question is how can one make sure no crap gets loaded again without barring him from using my imac again?

 

[MBAir2010]

simple math: naughty nefew + discipline = corner

 

[tromboneaholic]

Create a "standard" account for him in "Users & Groups" rather than letting him use your "Administrator" account.

Even better, have him use a guest account that will delete everything when he logs out.

 

[MacDawg]

Even better, have him use a guest account that will delete everything when he logs out.

This. A Guest Account is definitely the way to go if you are going to allow access at all.
Although, I gotta say, nobody gets on my Mac for a joy ride period.

 

[chown33]

A managed account is one step up from a Guest account. If the nephew might use the computer again, then that might be worthwhile. The hard part is doing all the setup using Parental Controls, since that can take some time and effort. In that respect, Guest account is much simpler, but also has no persistence.

 

[hobowankenobi]

As stated, a non-admin account is the first line of defense. A standard account or a guest account works.

I make a kids account for such occasions. They can't do much damage...and no damage to a "real" account.

Keep in mind some browser plugins and extensions, and drive-by installs, and stand alone applications do NOT require a a password, but typically only affect the account that they are installed into–not YOUR account. That's why a separate account is key.

I would consider the guest account, unless you want to spend a bit of time cleaning up the debris next time kids visit. It does make for a non-destructive (to YOUR account) way to learn how to clean out cruft. For cruft-free bliss, the guest account is the way to go.

BTW: You could have cleaned you the mess manually...but too late to worry about that now.