As has been said, some people sends out a lot of PMs searching for partners and might not remember all their partners right away when they see them, others might just want to roleplay with you again. Usually the second is easy to spot since most people say "Hey, want to roleplay again?" or something like that. If they approach you as if it is the first time though, then it's probably the former.
If they are bothering you too much, you can put them on ignore, but since that will make you unable to see anything of theirs (PMs, profile posts, forum posts) it can create confusing situations if you happen to get into the same group roleplay as them or you talk in the same general chat thread as them. The ignore feature isn't to recommend if you and they are very active in the same group areas.
As for reports, as Gwazi said, talk to them first and if they still bother you, it's just to send in a report and explain the situation and we'll have a talk with them. Make sure to save your messages so you can send them to us if we need to see them.