This will vary between jurisdictions. Some states allow "self help," which means that a landlord can remove a defaulting tenant without first going to court. In others, like my own state of Michigan, this is absolutely forbidden and can result in a lawsuit against the landlord.
I suggest that you go about this two ways. First, send the tenant a certified letter stating that the car must be removed and that you will treat it as abandoned if it is not. A reasonable notice period is necessary. I suggest two weeks. If you do not hear from the tenant, contact your municipality to report an abandoned car. Let law enforcement handle the abandoned property to avoid personal liability.
The second suggestion is to go to court - without knowing the state and the circumstances under which the tenant left I cannot specify a proceeding - and get an order to remove the car. Unless the tenant answers, this should take less than a month.
In the future, you should add a clause to a lease providing for removal of personalty left when a tenant vacates the property.