Hi,
Generally speaking, I can't imagine starting a business letter with 'To whom it may concern'. My preference would be 'Dear Sir'.
I think you'd perhaps need to look at the purpose of the communication in order to decide how to end it. It might be that a simple name ,or even no name at all, might be appropriate.
eg
To whom it may concern:
This space is reserved. Please do not park here again, or else your car will be towed away.
Building Management
I feel the phrase sometimes even has a mildly threatening tone, which does not seem to warrant much cordiality in the ending.
Best wishes, Clive