GovernmentAttic got a list [PDF] of all the vanity plate ideas the DC Dept. of Motor Vehicles has rejected, as of 2012. Here are some of the funniest:
ASSS
ALL BALZ
APESHIT
ASS4CAT
BANSEX
BATMAN
CATMAN
CTHULHU
BONGMD
BUGBUTT
DEJAVUE (this one presumably rejected by a spelling stickler)
DRUGS
ETABUGR
FAHRT
IMALION
JANET69
JOGSEX
KERMIT
LOGIC
LORAX
MERLOT
MFPLEZE
MORPHIN
MRAHOL
MUUFFIN
NAKEDROB and NAKDROB (apparently Rob tried this twice)
ONEASS
PENUS
POOPBUT
PUGANUS
SEMENS
WTFWJD