Emil,
Disclaimers aren't worth the paper they are written on.
The vast majority of clients accept why stains won't come out if it is explained to them properly and the ones that won't aren't in my experience people you want to be doing business with.
If I were you I wouldn't offer 100% guarantee on anything, especially stain removal as you are leaving yourself wide open to people taking advantage, plus, stain removal is not something you can guarantee as there are so many reasons why removal may be impossible.
People judge you on the quality of all of your work, not just one aspect of it.
Simon