Yep - just swap over the contents.
The other one is to simply put it back in the post marked - return to sender
I think if you open it and use the freepost envelope, Royal Mail get paid for delivering it back to the spammers. If you just put return to sender they have to send it back but don't get paid any more.
OK, they are sometimes a complete shower of incompetents, but I'd rather have them and their 'same price to anywhere in the country' around than the variable pricing structure we might end up with if they can't make a profit on what they do.