I recently quit my job (in MD) after only 9 months of a terrible experience (the job was nothing like what I had been told at the interview..different responsibilities, different boss, etc). I put in my two week notice and continued to proceed "business as usual" but was asked to leave after only working 4 days into my 2 week notice period as my employer cited "my duties had been transferred." However, when I received my last paycheck, I had not been paid for those 4 days that I legitimately worked.

I owe them a sign on bonus repayment, however, is it legal to hold my last pay as "ransom"???

No, it is not. But if you provide them with the bonus repayment you owe them, they won't have any justification to withhold your paycheck, will they?

In other words, no one's skirts are clean here.

While it isn't legal for them to not pay you, it makes a great deal of importance in MD whether or not you signed anything when you were hired or at some point in your tenure that addressed recouping the funds from your last paycheck. Perhaps it was a document you signed to receive the sign on bonus.