It is illegal to shoot down ANY aircraft, whether it's full-sized or R/C if you want to get technical and carries the same punishment. If you want to blow one out of the sky for kicks, then it's up to you. Take your planes over the the conservation club on the west side of town, they have a nice large field there where you could take yours down.
I highly doubt that an air-soft pellet gun would do much damage if you could even hit the plane.