Lately I have been getting the same thing in RIFT, strangely only from Defiant players.

Not sure about WoW as I never had the issue of being harassed but in RIFT if they do not leave me alone and continually /follow me and spam duel requests/whatever else they can do to annoy me, I just report them.
I then get a ticket response from Trion saying "we hope we have resolved the reported issue to your satisfaction", which is usually followed by an apology from the offending player.

If don't annoy me too much I basically say the same thing as Nightsham and / or "Good luck with that, please let me know how that works out for you."