How do I stop spammers hijacking my twitter account?

I don't use twitter very often, but I've just been into my account today and discovered that my account has been "tweeting" stuff I didn't send. I stupidly clicked on an ad on the sidebar of Facebook a couple of months back and it seems that variants of the company(ies) running that ad have hijacked my account somehow. Unfortunately, it's a different link they give everytime. I may have entered my webmail account (which I use for my twitter), but definitely not my twitter details. How should I deal with this. Do I need to deactivate my account and start from scratch? (not too much drama to do this, as I have only about a dozen followed/followers) Thanks for suggestions.