There is a product (Florel) that contains IBA indolbuteric Acid. This will prevent nut formation & must be applied when the tree is flowering...... It should be noted that the allergy is probably due to the flower pollen & this will not help... that being the case the solution is a chain saw.

You could always order a perfectly timed late frost coinciding with pecan bloom! There is no way except the chain saw to prevent it, as yiorges aptly states.
Unfortunately, in other species we are guilty of promoting more and more spring allergies by planting "seedless" varieties of many shade and ornamental trees. Why? Because they are MALE varieties in species that are dioecious (either male or female), males producing the pollen but not the seed. So we give up the mess of fruit or seed and have more allergic reactions. Pecans are monoecious (both sexes on the same tree) and will produce pollen seasonally.

I've used Florel without much success. I believe it is labeled for pecan, although you'll have to check labeling in your state. I'm not faulting the product. Timing the flower and getting adequate coverage are challenges. We're arborists with lots of experience and specialize in treating large trees but we gave up on this approach.

Hortster and yorges have the answer. If the reactions really are linked to the pecan trees, what moves through the wind is the pollen, most likely. I know of no product to stop bloom effectively, and even if one exists, treating a tree canopy with it and obtaining good coverage would be something I wouldn't want to try.