About 25 years ago, one of my then neighbours kept having his big ornamental flower pots nicked from his garden . He was a brute of a man, bearing an uncanny resemblance to big Pat Roach, so I'm sure the thieves didn't know him else they'd have been minded not to bother.
Anyway, he got so fed up with is that he glued a load of stanley knife blades around the rim of one of the pots. Sure enough, a few days later one of the pots was found to have moved a few metres, and then an impressive looking trail of blood led off into the distance. After that no one touched his plant pots again.