Call your agency and ask them to intervene. Tell them if they don't get you out on time, you are requesting to be paid for the time you are delayed.
Either that, or complain to the supervisor, and ask her to sign your time slip for staying later than scheduled. If she won't, then ask her to come up to the unit to be present and se that report starts on time.
You deserve to be paid.
I will ask, before you do any of this, have you tried the direct and obnoxious Hoolahan approach? Walk up to the group, and loudly say, "OK people, grab your coffe, I have to get out on time tongiht, and I will be starting report in 2 minutes, with or without you." Sometimes I have said, "OK guys, I have to get out on time, so if you don't mind catching up on your chit chat later, I need to begin report right away." I am not their to win any popularity contests. I always work hard, and help out other nurses who need help if I am caught up, etc... They never have to ask me for help, if I am free, I just go in and start helping if it's needed. So, they are a little tolerant of my antics!