Any costume with fake abs
If girls have to live off of Fit Tea and Adderall and take their workout regimen up a notch so that they can look good in a slutty Halloween costume, you should NOT be able to wear fake abs. It's so not fair. Also, pretty much any costume that involves fake abs is an adult version of a kiddie costume. Superman? Power Rangers? Thor? All little kid costumes. Girls can wear kiddie costumes and get away with it because they've worked so hard for their hot bods to wear costumes that have as much fabric as kiddie costumes.
Sidenote: Got this pic from Donald Trump Jr.'s Instagram. This guy has such bad daddy issues that he definitely wore slutty Halloween costumes when he was younger.
[Photo via @donaldtrumpjr]