Just tell them you both feel it's not going to work out, wish them well, and say "Take care." If they get in touch again, ask them not to contact you anymore.
Did you give them your real numbers? Or a Google Voice phone number? I love GV, it forwards to your phone(s), receives texts and voicemails, and you can block people, who never had your real phone number to begin with.
If you think your ads are being flagged on CL, maybe start re-wording them so it looks like they're coming from different people for a while. They might not think it's you. Or maybe there's just someone going around flagging couples ads.