I don't think you can. It is basically free software so as a provision they just ask for their link info at the bottom. I think you may be able to remove the "Powered by..." portion, but you will have to write them about it. Check here for the official word: SMF Copyright Terms and FAQ.
Well you can try this: Am I allowed to remove / change the copyright?
From that page:
Comments: No kidding take a look here the impossible has been made [..] a good tutorial how to remove the SMF copyright / footer works 100%!
Comments: When you create a custom theme do you leave the SMF copyright in there or do you take it out?
We do not modify the SMF copyright in any way. I have had requests for this in the past but always refuse.
We would not like for people to remove our design by link in the Themes we create so in the same way we do not want to dishonor the SMF creators by doing that.
If you need a Theme you can always check us here: SMF Custom Themes.
I can't see how you can use something that is given to you FREE and you still want to take away any credit from the person who gave it you for FREE. Come on, give credit where credit is due and leave the SMF copyright there.