You can lay landscaping cloth and cover with mulch. This will keep the seedlings from sprouting and will also stop other notorious invaders such as poison ivy.

Some maples are notorious in this regard, Christine. On a mulch pile you can certainly use an herbicide like Roundup. In the middle of a bed, I'm afraid you don't have much choice but by hand. Maples tend to have a tenacious taproot even when young, so I've found that using pliers to grab right where the plant comes out of the ground makes the task go much more quickly.

weed -wack till they die every week!!