I read somewhere that most of them had been warded by Rand or at least he had ordered someone to do it (around book 8-9) but it was also pointed out that whatever wards his people set could easily be undone by a forsaken or an equally skilled channeler.

So my question is, especially with all the problems the good guys have had with trolloc army's coming through Waygates, and to a certain degree I guess Portal Stones could fall into this same category, the fact that they're corrupted and hold Machin Shin, and finally the fact that they don't need those things except for maybe study since they now have traveling, why haven't they destroyed these things?