If you are at war, then using explorers to pillage key resources is an excellent tactic, and one that I heartily support. If you are at peace and are keeping an explorer on top of an AI resource so that they can't use it, without a right of passage agreement, you are exploiting the AI. This is a shady tactic that I consider an exploit, simply because the AI is not programmed to deal with this situation. If you want to use it that is your business, but I don't consider it to be a fair tactic. Note also that this will not be allowed in the next Game of the Month competition (the June one).