Go into the source hive and see if there are any queen cells left. If there are either destroy all but the best looking one or make some splits if you have the extra equipment (this would be my choice-I hate destroying queen cells). They will probably keep swarming as long as there is more than one queen cell left. You could try adding a medium and mixing drawn comb with the other comb to give them more room. If the topmost medium if full of nectar/honey (or the second and topmost) they think they are out of room and swarm. Good luck with the hive!