I guess you can do it as for other varieties?
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)
- Plastic beverage or food containers (cleaned).
There is also a more complete explanation on how to do at http://howtosaveseeds.com/isolate.php
I think the surest way is Bagging and hand pollination... But it is much work.
If you just want to avoid buying seeds every year
If you only grow one variety to flowers, and your neighbors don't, then you can probably take the risk to grow one variety of seeds per year by just leaving the plants as they are. This assumes most of the seeds will give the same kind of plants, and some will have been pollinated from far, but only a few.
However I know you cannot do that too many times, and must buy new "commercial" seeds. Leave this hard "hand-made pollination" to small biological farms, and buying seeds to them will help them sustain.