Contact the community resource officer in your police department and have him meet with the group to address the problem. It shouldn't be a voluntary meeting, have the officer show up where to ride meets and tell the group in no uncertain terms of how the cow is going to eat the cabbage. Weekend mornings are slow in most small to medium sized towns. Let the group know that officers are going to stop the group should violations be observed and the riders ticketed. End of story. No warnings. If your city has a bike unit, you might talk to the head of the bike unit as well. But it doesn't need to be a feel good meeting...a stern warning about what IS going to happen. I bet the PD would like to know when and where this ride begins because more than likely they have gotten complaints about it.