Don't think so, but you can rename it. Not as good, but it does make it less obvious. Call it something like guest_user_testing or something like that and give it a rediculous password that even you don't know (3 guids cast to varchar and put together work well)

As I said before, no amount of tweaking sa's properties will help you here. You need to tell the user that the cannot have the sa password of sysadmin privilidges. End of Story.