Towing Issues: Towing from a private parking lot, apartment in washington dc, illegal parking

Question
I rent an apartment in Washington, DC, as well as a spot in the apartment building's small parking lot (8 spaces).  Those of us that rent these spaces have verbally-assigned spots ("Yours is the second on the right") There are no signs posted anywhere in the parking area regarding "Private Parking" or "Reserved Parking Only" and certainly nothing about towing violators.  The real estate company that owns the building has told me to just call a tow truck if someone else parks in my space as signs are not needed in private parking lots.  Needless to say, my spot has been used without my permission numerous times.

Questions:
1) is it legal to simply call a tow truck without any signs posted?  Doesn't the owner of the parking lot need to be the one to call them?
2) would I have to pay any money to the towing company to do this?  As far as I've been told, the landlord does not have a previous arrangement with a towing company.
3) how would I report this--as an unauthorized vehicle?  an abandoned vehicle?  
4) do I have any way to make my landlords post signs to prevent this from happening again?  I'd rather prevent it then have to call a tow truck 3-4 times a month.

Thank you

Answer
Wow, I would really hate to be in that place as it is ripe for misuse and problems.

First of all as a general rule, the property owner / manager is the one that is authorized to have vehicles removed from property for "trespassing" or "illegal parking"

But in a quick search for rules and regulations in Washington DC I came across the story and information below...

I am not sure if this is current and accurate but its a big start toward finding out the information you are looking for.

http://www.dcwatch.com/mayor/020220b.htm


It appears that there is a government department which regulates towing even from private property and I see a reference to the police having to issue a ticket...  I am kind of at a loss for information here as I see a few things which are normal and some which are totally different...  

Hope the link helps and points you in the right direction