I would consider it flattering if I had a stalker. Well, a stalker who's stalking me because she's interested romantically. Buuuuuut since I'm taken... Politely decline, or do the unpredictable thing and become good friends with my stalker.

But I'd make it very clear that I am not interested, in order to not 'lead her on', as it is.
Of course, I would be absolutely thrilled if the person whom I'm taken for stalked me. Both online and in real life. It shows I'm special enough to get special attention.

They aren't really stalking you if you like them.
I don't there's anything flattering about some creep who can't get the message sending you abusive emails for 6 months.