I was told by the police here always to report. Reporting a person for aggressive behaviour establishes a paper trail, and makes it easier for them to prosecute if they have to. The last time this happened, which was a few weeks ago, the man in question was having a psychotic episode, stole from us, and I ejected him from the premises and reported him. He later assaulted someone in another bookshop. I understand that the police have picked him up, and there's a record of his behaviour. If someone molested me at a convention, I would take an exceptionally dim view of anyone who suggested that I refrain from calling the police.