As you say, it appears to be a kind of paint? So perhaps if you get a razor or something sharp you could try scraping and it will probably come off eventually, that would be one physical way. On the other hand if you want to go crazy try putting some paint thinner on there and see what happens, but don't blame me if you ruin your shirt experimenting on it, I have never used paint thinner before but I am guessing it consists of a non-polar compound and dissolves similar compounds (paint) so that it might be washed off with water. This is all hypothesis of mine, I have no idea what will happen except for what a razor can do.
Edit: I believe that Mr. Bean may have already demonstrated this experiment for us.