Often comes after something said by another person.
The word "Do" In the meme is commonly changed to whatever the topic relates to, Like "Can't let you eat that." Etc, Etc, Etc...
Usually used in sarcasm or to be annoying. Most people don't take it seriously, because it's really not.
n00bs will usually go all "N00b-Fu" On you if you use it at the wrong time though.
Protect your e-Crotch.
Random guy: CAN'T LET YOU SHOOT THAT
Guy (Talking to another guy): So, I'll meet you at the cafe in ten minu-
Random guy (Interupting): CAN'T LET YOU DO THAT!