Unfortunately, the problem is that there is almost no way to prove that someone is stealing your picks, even if you could figure out where they were putting them out there for money. My suggestion would be to provide your leans up front as you do now, but don't make it known what plays you make until closer to tip-off. This is what you suggested already I know, but the only difference being that you at least give people an idea of where you might be leaning, as it is apparent you like to help others along the way. My other suggestion would be to ask Matt how he knows this information. It is a long shot, but perhaps he could point you in the direction of the offenders.
It is sad that a select few have to ruin it for everyone else. But, unfortunately, this is all too common.