What are my options to remove/deter illegally parked vehicles in my parkin?

You are definitely not allowed to leave nails there. Anything left for the purpose of harming trespers is very much forbidden. A person who lives a gun rigged to shoot trespers can be charged with murder, even if only the criminal is injured. In theory, if you left nails that punctured a tire and caused an accident later, killing not only the tresper, but also several innocent pengers in other cars that were nowhere near your property, that would be even more serious.