What should I do if a Yahoo user is stalking me? I did something stupid to them, but I already apologized, but now they are doing it to me.?

If you Apologized thats about all you can do the Best thing to do at this Point is to just Ignore

depends on the deranged  level of the troll. 

They may accept the apology-------they may get bored after a week or a month.......

or they might continue to stalk the living crap out of you  , day in, day out,  year in, year out,  like my stalker has been doing to me for 4 solid years now. 

Ignoring them is the best advice.....when they see they can't get a rise out of you........they'll give up eventually and pick another victim. 

Make your activity private so it's not easy for anyone to see all your posts and either post harassing comments toward you or report your posts en masse. You can make your activity private by going into your YA activity, selecting Edit Preferences beside your avatar, and then choosing "visible only to me" for your posts/activity. Otherwise, you can report violations, but there's no much you can effectively do to stop someone from simply posting where you post or reporting your posts (because you can't really know who or how many users may be reporting you, if that's an issue you're having).

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Unless you fear for your personal safety, make your activity private and then ignore anyone harassing you. When they don't get the desired reaction, they'll eventually tire of pestering you and move on. Every time they get a response, it just encourages them to continue their behavior.

Post anonymously.

I would not stay off this site just because someone is trying to stalk you.  But you can change tactics.  Try changing your name or going anonymous.  If you can block them, but most important ignore them.  Never feed the troll.  You show you are upset with them they win.  It was nice of you to apologize to them but they may look at you as being weak.  Still, the best is to make believe you do not see a thing they post about you.

Stay off YA for 2-3 weeks so they will get over what you did.