There's an iPhone app called Glitche that does this. Https://glitche.com/. Source: about 3 years ago

The sign idea is smart if you could put it in a place that's very visible but can't be grabbed by the neighbor. Https://shipyourenemiesglitter.com/ is an option Maybe there are free newspaper/magazine subscriptions you could sign them up for to receive in the mail. Source: about 1 year ago

Others ship shit or a glitter bomb for them to open after they steal it! The shit senders are in the US but don't ship to Canada - there was a local one in Caledon, but they've stopped last year. Basically they box up a pile of horse manure, cow shit or zoo shit then mail it to your enemy. As fun as that sounds .. I think the glitter will be more evidence! Source: about 1 year ago

Can get one of those cards filled with glitter. Companies like Shop Your Enemies Glitter provide such a service. Source: about 1 year ago

Envelopes full of glitter. https://shipyourenemiesglitter.com/. Source: over 1 year ago

Here you didn’t ask for it, but it is what you are looking for. Source: over 1 year ago