About this phrase:
"Are there any services we list which don't work with Free Software?"
I would extend it to:
"Does they make the visitor/guest/user use non-free software through JavaScript?"
If the answer to the above question is "yes", then mark the button with a special color or symbol, with a nearby note on the issues. No need to entirely block/remove the button, just mark it specially. I think even the FSF does this if one clicks the "Syndicate" links in each blog publication.