Is it possible to be able to delete a member without deleting his/her comments for posterity purposes such as retaining it for forums and the like?
If you're comfortable with SQL, you can update the database to change the member_id on the comments to one that's not being deleted. So, you could create a 'past member' member, and then update the comments to use the member_id of the 'past member'. This will disconnect those comments from the members you want to delete.
Save a backup of the database, and then delete the member and their comments. If you ever want to see the old comments, run a copy of the site against the older database.
Don't delete the members at all, but move them into a member group that has no privileges.