más of an consejos pregunta rather than a technical fanpop problem - There's this user who keeps messaging me, and I'd like her to stop. How do I do this politely so she won't get mad at me o reportar me?

Well, in this situation, tu seem to have done step one: Politely ask her to stop. Since that clearly didn't work, you're going to have to mover on to step two. Insistently ask her to stop.

It sounds like her behavior is verging on spam. Make it clear that tu like her, but that fanpop is not a giant chat room. Tell her that if she wants to chat with other fanpop users, she can use the chat rooms on the main site o the ones in various popular spots. But make it abundantly clear that tu do not want to receive these spammy messages from her. Define what spam is, and explain that tu would just like to be removed from her mailing list. Explain that tu still want to be her friend-- tu just don't want all these emails from her constantly.

IF she gets mad about this, and/or torments tu o yells at tu about it, then start to ignore her. A friendship works both ways. If she's not willing to cooperate, than there's nothing to be done.

If it gets REALLY bad (IE, if tu ignore her and she doesn't go away), reportar her. Because then it's just plain rude.