!denybuild is enough. Greifers will find ways to abuse ANY system, because if you lock it down too much you end up hurting the good players more than the decon would have in the first place.
Case in point is the patch preventing you from deconning the last spawn. It's obnoxious, especially on ATCS as human, instead of deconning nodes and moving them, I have to first decon the armory or turrets to make room for a 2nd node, then decon the node I wanted to move, build it where I need it, then decon the other one. I've had this happen multiple times, as it comes with one base design I use. I know markdecon would make it easier, but it too has its bad points.
On a secondary note, I would like to see a patch for markdecon that allows old-style deconning along with the new. Markdecon is usually better, but there are times when it really sucks. Perhaps a different command? /decon_now or something.