>> cross-pollination
Bagging? That's tedious, and small-scale, but it works.
You could wait to see which variety goes to seed first or most vigorously, then uproot everything else of that species. Then pick all the fruit from your seed plants for several more days to purge cross-pollinated fruits. Then save seeds from the next round of harvests.
Of course, you might have a dozen varieties of some species like Brassica rapa
Have you considered saving mixed-parentage seed from year to year? You would not have the original heirlooms that you chose, but the hybrids will be selected for suitability to your micro-climate and local pests. You would probably have to work each year to find the plants that gave the fruit you like best, and only save seed from those. (It's also good to "rouge out" the worst third of any crop, if you are trying to steer a hybrid mixture of genetics into the particular traits that you want.
That approach is like making a mini-landrace.
