Possibly not much. This is a common bugbear for tenants. Although not 100% clear, im going to assume you have been granted the right for two particular car parks and the landlord has the right to do so (i.e. they’re either in title or been granted by exclusive use).
The first step is to have adequate signage to show that those are your reserved parks. This doesn’t stop people parking in them, but may dissuade some people from doing so. You’ll need to get landlord approval to do so, and possibly the body corporate’s as well. It doesn’t work on everyone - which I know from experience. I used to work above a child care centre. Parents were always parking in our reserved spots despite other car parks being open, and then making up all sorts of excuses when challenged.
Have a talk to the other business if you can identify whose customers are using your spaces. Explain that you’re paying rent for these spaces and being denied the use. You can also raise it with the landlord. Both parties’ reactions will largely depend on how sympathetic they are. I doubt you will have a mechanism in your lease to decrease rent.
The only other options are to make the spaces too hard to use/calling out the people. Doing this is up to you, as the range of reactions you can get may be aggressive: Make sure you or someone you allow take up the spaces before anyone else can get them. Leave notes on windscreens. Park people in while leaving a note on how to find you to get out. Be prepared for reactions from honest mistake through wilful ignorance to spitting attack (and, unfortunately, the possibility of physical violence). People hate being told they can’t do something, and generally hate even more being caught doing it.
Unless you can get the whole centre to agree, and are very careful about the way it is done, there’s very little chance of being able to have the offending vehicles removed. I wouldn’t bother going down that path unless it’s a very common problem to all tenants, and the stars are aligned.