This is a common situation, and the same rule applies here as everywhere else - start being nice, then you have 'nasty' to go to - if you start a war of attrition, then you will lose every time.
Start with a polite note on the windscreen - 'Please park with courtesy for the benefit of all residents' - nice and friendly, non-specific, non threatening, will hopefully work.
If not, a polite word at the door, do not get into an arguemtn, remember, never upset someone who'se co-operation you need.
If these matters fail, look at your legal rights as a part-owner - do you actually own rights to a parking space? If so, you can engage a solicitor to send letters advising that you are going to embark on legal enforcement of your parking rights.
Good luck.