The problem is that the freeserve e-mail address is also your account address and the bit after @ cannot be changed.
I found that quite a lot of spam wasn't using my name before the freeserve account details but made up rubbish. I used their webmail filters to move some of the more commonly junk mail into the delete folder on webmail. This is not downloaded to your e-mail reader and is automatically deleted after 7 days.
Article Orange Help kb 2171 why am I receiving someone else's emails? states how to set up thses filters.