A person that is usually described as soaking up all the fun out of a room or situation. They are generally quite boring, lame and humourless people and don't allow those around them to have any fun.
Person 1: Hey is laura wagner coming to the party tonight??
Person 2: No I told tara not to invite her, she's your typical type of fun sponge. I mean I do want to have fun tonight don't you?