Smart Matches cannot currently be deleted.
Rather than deleting matches, you should confirm or reject them.
Rejecting gives you the opportunity to review matches later as your research progresses, even if you decided they weren’t good the first time around.
It sometimes happens you’ll receive a match that on first look seems completely wrong and then turns out to be correct at a later date. It would be a shame to lose that connecting thread if the match had been deleted in the first case.
Rejecting, rather than deleting is meant to help you retain research leads that can advance your work when you get stuck.