This is very typical behavior for a male guppy. They'll try to breed with anything, whether it's of the same species or not. Are your platies girls? Try providing lots of hiding spaces, however, this is the male guppy way and not likely to be changed or stopped. What size tank and how is it stocked?
My male guppies will not leave my platys alone! They are constantly chasing one of them and nibbling on its fins, the man in the shop ensured us they would be fine with platys but now I'm not so sure if I should keep them, any suggestions?