Who do you look at onscreen and think "jeez how did you make it this far". If the actor has charisma but can't act that's ok and they are to be forgiven (eg Arnie), we're looking for those wooden actors who are just so samey no matter what the role.
Here are my choices in no certain order :
Ben Affleck
Drew Barrymore
Madonna
Robert Redford (controversial one I know but his characters from most of his recent movies could be the same person)
Martin Lawrence (needs to be funny to disguise his bad acting but he fails there too)
Jared Leto (he was so bad in Panic Room I felt sorry for the guy)
Quentin Tarantino (excellent director though so a tough one to list for me)
Woody Allen (without doubt the most samey actor ever)
Jean Claude Van-Damme (nuff said)
Mike Myers (even his voice over in Shrek was boring, I've had funnier vommiting sessions)
Pierce Brosnan (oh cmon, you know he's only suited to one type of role, watch Dante's Peak if you think he can be anything but Bond

)
Stevan Seagal (Under Siege series was ok but his other films just scream of "I'm A Macho Man, Don't Mess With Me"
Jim Carrey (see Mike Myers

)
There are probably more but that's what comes to mind immediately, only two women in the list, there must be some more surely
Regards
Phil