Select the objects you want locked, Press right mouse button and choose lock.
Adjust the terrain.
You can unlock them by holding the spacebar and moving the mouse over the locked objects.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.