YAF is distributed under GPL v.2 licence. According to it you can use YAF source code as part of your commercial support forum, but you can't redistribute YAF in any modified form with your commercial products.
According to GPL v.2 terms you can remove credentials, but developers will not be happy at all at this, because YAF can only be well developed when it's becoming more and more popular. The link to the project site in the bottom makes the trick and we expect that you'll do it using YAF. The logo can be removed without any problems.
If you want to discuss special terms of usage or unsure you should contact Jaben
for more info.