Oh dear, who would do something like that? Those really are malicious people and apparently they have nothing better to do with their lives. :|
Now, I think the safest thing to do is make another account and repost your stories there, though that could lose you subscribers and friends who might never return. :/ If you wish to keep this account, then the most important thing to do is to change your password. That is crucial. Just like heclehog say, choose something very long and complex, preferably a mix of upper and lowercase letters, numbers and signs. Make sure it isn't something too simple or easily guessed from your activity on AFF. It's also advisable to have a different password for each account you make, instead of using the same one everywhere. And yes, changing your username can help as well. I will send you some points if you need them.

how about making an new acc instead ? transfer your karma there ... and then del this account

Try reporting it and get help from the mods? Hopefulyl they have a way to help you..

I suggest you talk to one of the mods here on AFF if this happened.

Sent you some points ^_^

Start commenting on lots of blog posts and stories I guess.... also, make sure you've changed your password to something VERY long and COMPLEX that way they wont be able to get in again. That really man :/