This situation came up recently at our church. We had an online registration using a Master Org and the Registration Type User Selects. This presents a list of the ‘pick list’ orgs so registrants can select the one they want.
Some people were registering for one org, returning and registering again for a different org. They all meet on the same day and time, so it is not possible to attend more than one. We want to keep them from registering for more than one.
Other Org ID to Join feature on the Registration > Registration tab of each of the orgs in the pick list. Enter the org ID # for the Master Org (the one in which you normally do not want people enrolled). When someone registers, he will be enrolled in both the pick list org he selected as well as the Master Org. Then, if he tries to register again and make another selection, he will see a message that he is already enrolled.
Note: If someone wants to change his selection he can contact the church. You can move him to the other org. Or you can drop him from both the org he selected as well as the Master Org, and he can register again.