Question
when is the best time to prune and clean out oak trees to minimize  acorn production.... the acorns are almost too much for me... there were so many this year it has become dangerous to walk on them... had to clean them up in some areas

Answer
Pruning will not work since this over time increases the foliage production and the flowers. There are products that can be sprayed on the tree during flowering to greatly reduce the acorn production.

What needs to be done is stop the flower formation. It is possible to spray the oak each year with a plant growth regulator to prevent acorn formation An ethephon spray during flowering is recommended to prevent acorn formation but correct timing of the spray is critical.

A garden center may sell ethephon under a trade name such as Florel. Ethephon breaks down to release the plant hormone ethylene. A homeowner may find it most efficient to hire an arborist to spray their trees because an expensive sprayer is required for tall trees. .

The good thing is the production of  acorns will vary greatly by year depending on the weather (frost usually) during the time they are flowering.

Here is a link to the web for Certified Arborist--I would only use one that is certified.

http://www.isa-arbor.com/findArborist/findarborist.aspx