It seems you have an acceptable use policy without a policy for dealing with unacceptable use.

Check with the library's legal counsel. If your library doesn't have one, then you need to find out what legal counsel you have for anything that arises.

Check with resources available through ALA or your state library board, discuss this with your board, & instate a policy for dealing with this when it comes up again.

To those who are suggesting filters: THEY DON'T WORK! Ask any Mudcatter who has to deal with them. Look here. Filters block perfectly legitimate material while letting questionable through. Porn sites should be shut down, but in this virtual world many are beyond US jurisdiction.