Just out of curiosity, why do you need to do this? You're looking for a funky, muted tone. Okay. That aside, what needs to be muted? The string you're slapping/popping? The other strings?
The only string(s) that should be ringing is the one you hit with your thumb/one(s) that you pop with your finger(s). If you're having trouble with this, slow right down and work on the accuracy.
If you need to silence the slapped string before a pop, I think most players use a sort of rolling action with their slapping hand. They palm-mute the slapped string and that silences it, then kind of rotate their wrist clockwise (for us righties) and either pop the string or slap again.
At least, that's what I do. I thought I paraphrased that from Slap It!, but in there it says to use the heel of the right hand. You strike the strings on the downstroke with the thumb, and on the upstroke you allow the heel of your hand to touch the strings just enough to mute them.
If you're talking about the actual sound of the note, mute with unused fingers. A great example of this is FRP, from Tower of Power (see
here . Not slap, but funky!) The whole finger mute thing is what goes on there.
You probably knew most of that already, but I hope that helps!
PS: Check out Fingerstyle Funk by Francis Rocco Prestia and Slap It! They'll both help a lot, I think.