Prevent unauthorized parking

[Ray S.]

I have a 4-plex with 4 parking spots. People keep parking in our vacant spaces or blocking the driveway. The tow companies never come when I call them. And I'm also slightly afraid of a retaliation if I have someone towed, which is why I'd like to focus on prevention first

What can be done to deter or prevent people stealing spots? I was thinking about installing folding, lockable bollards, but I'm not sure I like the appearance of them. Anyone recommend them or have any thoughts on other prevention methods?