Move them away from each other varieties.
Put a bag over a specific branch after hand pollination and mark it.
Put the variety you want pure in a different room.
Basically, you do not want to take the chance of pollination mixing across all the different varities IF you want to try to keep the seeds true.
However, even if you take these measures you may still not get true seeds but taking precautions will give you a better chance.
Think of it like being in a room with someone who had an infectious disease. What would you do to isolate yourself from that?
Good luck!