I've got the same problem, only the tree is in my front yard. Because I've planted shrubs and perennials in a huge bed around the maple, I can't use chemical herbicides on the seedlings. I hoe them up when they are small. If they survive within my plants and get deeply rooted in, I use pliers to yank them out one by one. It is a really annoying chore! Since you don't have anything planted under your tree, you can 1-mow the seedlings repeatedly or 2-spray carefully with glyphosate (Roundup) or brush killer. (Don't let the product drift onto surrounding plants.) Best of luck.