Help, my url has been hijacked!

I've spent much of the day deleting undeliverable messages from spam filters all over the world (many in Chinese). Spammers have hijacked my website address and used it as the return path on their e-mails. I'm getting about 10-20 messsages a minute at the moment. Fortunately 99% of them are picked up by my own filter, but it's still annoying as occasionally the filter picks up genuine e-mails from clients so I have to skim through them instead of deleting them en-masse.

My website hosts say nothing can be done and it is just "my turn" today. Eventually the spammers will stop as they never use the same url for long.

But there must be some way to stop this from happening - short of suspending the url altogether. Does anyone else have any experience of this?