I guess you can do it as for other varieties?
http://seedworld.com/cross-pollination-8/
Cover the flower heads.
This technique (also known as "bagging") will need to be done before the flowers open so that bees and any other pollen transporting insects are preventing from getting to them.
For covering the flowers only, try such items as:
- Toilet rolls
- Muslin or similar sachets or bags (these can be tied in place)
- Handkerchiefs
- Plastic beverage or food containers (cleaned).
There is also a more complete explanation on how to do at http://howtosaveseeds.com/isolate.php
Hand pollinate
I think the most sure way is Bagging and hand pollination... But it is much work.