Unfortunately, all the law requires is that the blue handicap sign be properly in place. If the spot is properly marked as handicapped, then any vehicle parked in that spot must be presently displaying the placard designating the vehicle as handicapped.
I completely understand your perspective; that being that everyone knows the car belongs to you and that you are handicapped, as such, they should have left the car. There is no doubt that this would have been a perfectly normal and acceptable thing for them to have done. Unfortunately, the law does not require this of them. The law merely states that the car must then be displaying the handicapped placard.
My only thought would be to attempt to calmly and politely explain the situation to the property manager. With this in mind, they might be agreeable to some very short-term payment plan to get fully caught up on the rent. Absent that, while they have been less than kind to you, they are within their rights to have acted as they did. Sorry.
I regret that my answer is unfavorable, but please understand that it would be unfair to you (and unprofessional of me) to provide you with anything less than a truthful response.
Also, several customers have asked how they may direct a question to me in particular. If you specifically want me to assist you in your legal matter, just put "FOR JOSEPH" in the subject line and I will gladly pick up the question as soon as I am on-line.