A nerfstick is the tool by which a moderator or admin on an online forum strikes down on bad posts and posters. It is similar to, but has more uses than the ban hammer.
This use of the term seems to have originated from a moderator on the Secret World forums.
Sappho: "I have a very large nerf-stick at my disposal, and I'm not afraid to use it"
User: Where is the nerfstick Sappho? Are you getting soft on us?