Greetings Heidi:
In order to prevent your apple tree from fruiting, you need to inhibit the pollination of the tree's flowers. There are several products available out on the market, though their use and effectiveness is sometimes hit or miss.
Florel fruit eliminator is one such product. A well-timed application must be made to your tree(s) to inhibit pollination. This product may be purchased over the Web or at local home/garden/nursery stores.