Acorn production varies from year to year -- sometimes you'll get a bumper crop, and other times just a few. There isn't a simple way to prevent the acorns from forming. Floral growth regulator can be sprayed on the trees when they bloom to prevent acorn formation. But timing can be tricky for the average homeowner to detect. (Oak flowers aren't showy like those of crabapple or cherry trees.) And even if you get the timing right, the spray must cover the entire tree, or the portion of the tree missed by the spray will produce some acorns. 

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Home spray equipment typically won't reach to the top of a mature oak tree. That means you'll have to hire a professional tree service to do the job. The spray is effective for only one year, so it has to be reapplied annually. Over time, that adds up to a significant expense. If mowing under your oak is a problem, you could mulch under the tree so you don't have to worry about whether the tree develops acorns. (And you'll spend less time mowing each week!)