Scratch

The easiest way I've found (using x and y velocities) is below.  Though you use direction to change these values, the technique should still work (it would be easier in my - uncommon - opinion if all your walls were the same color, which is how I've written my script)

when gf clicked
forever
calculate velocities
change x by (x-vel)
if<touching [wall v]?>
set [x-vel v] to ((-1)*(x-vel))
change x by (x-vel)
end
change y by (y-vel)
if<touching [wall v]?>
set [y-vel v] to ((-1)*(y-vel))
change x by (y-vel)
end