First of all, think whether you're legitimately offended, or you're just being too sensitive and politically correct.
A very good way to check it is to generate an equivalent but emotionally more neutral scenario and observe your own reaction. For example: London was heavily bombarded during WW2. It rained bombs, and a huge amount of people died there. Now imagine somebody posts the following joke:
'me and the boys running from the English Librarian after we put a book on London in the climate section'
Do you think this joke needs a "(warning)"? Do you think you should reprimand this person? If you don't feel terribly offended about this joke, probably you should ask yourself why you felt so offended about the other one.