@Doq If there are no other installations you wish to preserve on the drive in question, you should be able to simply create a new partition table from Disk Utility on an OS X install disc by going to the 'Partition' menu after selecting a disk, and then choosing '1 partition' or '2 partitions', etc., and then applying.
It has been a long time since I've last installed Linux on a PowerPC machine, so I don't explicitly remember if Yaboot installed itself into a given partition or the current partition table itself, like GRUB. However, if it's the former, you should be able to just look for any seemingly used partitions from Disk Utility's aforementioned 'Partition' tab, and then just erase them and repartition the disk accordingly. If you opt to only erase them while retaining the existing disk structure, that should then leave any pre-existing installations of OS X intact, bypassing the need to reinstall.