Put a couple signs up facing the street. Liberally smear some roofing tar ( or dog crap if handy ) on the back of the sign near the top.
When they run up and grab the sign it will get all over their hands, which they will immediately rub on their pants, not knowing what it was.
One treatment cures most sign thieves for life, especially when they have to get back in their car covered with tar or dog crap!
I lost a couple of signs to thieves during a special election here. The replacements were surrounded with relatively fresh examples of canine excrement, just so the next thief could mess up their shoes (and hopefully car and/or home carpet). The signs stayed put.