Mel,

down the bottom of the page, where the Quick Reply box is, on the bottom right hand side of the box you type in, it has a tickbox with the words 'show your signature' next to it. You just need to untick this box before you post your reply.

If you are in advanced mode, you will have again have an option on the bottom to untick 'show your signature'.

It's not that hard to do, so if anyone still doesn't understand how to do it, please PM me and I'll explain it again.