Is there a trick to dealing with terrain spikes?

[SwoopGB]

4 player multiplayer game going on and one of the lads likes building things very much, messes around with the terrain a lot then decides he's made a mistake and maybe shouldn't have planted trees all over that corn field and dug a pit next to the wood chipper to empty it into.....and unfortunately, that's just one example.

However, when he tries levelling the terrain back out again we're finding that sometimes they'll be a lump or spike in the terrain that refuses to be moved. Anyone know what's causing it and what we can do to fix it?

We did suspect one particular spike had a tree stump in it.