Thanks for the Question to Extension.  There isn't enough detail to give a positive identification.  Can you provide any more information such as when they appeared, leaf shape, color during summer and fall?  Had there been a tree growing there previously?

If you want to kill any that reappear, a broadleaf post emergent herbicide such as 2,4D, or carfentrazone should do the trick.  You could also use glyphosate but be warned that it will kill any vegetation it is sprayed on.