I have always held that if one has grounds for a complaint, or thinks that one has, a politely worded written or verbal complaint is in order, but that it must be signed with the complainer's name and address.
If I ever receive an anonymous complaint, which to date, I never have, I would take no notice of it. Formerly I would have put it on the fire or into the waste-paper basket. Nowadays, where stalking has become so popular a pastime, I might well keep it in my desk drawer until I felt certain that it wasn't the start of a harassment campaign that I would want to show the police.
Being anonymous on this or other online forum is a completely different thing in my opinion. We are usually exchange views and opinions on matters that interest us, not trying to complain or bother each other, and the site moderators apparently delete contributions that they feel overstep the mark.
If the communication was in any way threatening, it would go straight to the police whether it was signed or not.