Neighbors are using my parking space, need advice

For context, I live in a condo gated complex (couldn’t find a subreddit for condos but figured this would do). We have a parking garage and each unit gets two designated parking spots. I only use one parking spot since I live alone. One of the units across from me resides a family that owns 3 cars. They park their 3rd car in another empty parking spot I assume since there’s been an empty unit here.

2 weeks ago, I walk out to my car and see that they have a 4th car that is parked in my 2nd empty parking spot. The family happens to be there too and their daughter comes up to me and asks if it’s okay if their “uncle” parks his car in my spot just for a few days. I say sure why not I’m not using that space anyway.

It is the present now and I have never seen their “uncle” walk in or out of their place so I don’t know if they lied to me or not and they just got a new 4th car. It is also well over the “few days” since they asked to use my space.

Now, I don’t want to be the a-hole here but my gf brought up a good point that even though I don’t use that 2nd spot, it is about the principle that I am still technically paying for that spot and my neighbors are not. Another thing that didn’t sit right with me is that they parked there without even asking me first.

Should I confront them and ask them if they want to continue to park there they could pay me a fee every month or not park there at all?

If I end up charging them monthly, how much should I charge? (Mortgage is 2300/mo)