I've talked to the BPD at length about this, and you know what they told me to do? Call 911. I'm not kidding. I still feel a bit weird about that, so I have the Allston, Cambridge, Somerville and Boston departments on speed dial, and when I catch someone in the bike lane I call it in while perfectly visible in their rear view, and then I tell them they have about 3 minutes until they get a ticket. I've gotten a wide variety of responses to this, none favorable, so at this point I just inform them and move on. One of these days I would love to have the time to stay behind and see if the cops actually DO come and ticket them.