There are many players who consider this an act of war....and it is for sure in bad taste. Are these your CEO corporations ppl are buying, or corps in your country?
If it is CEO, two can play at that game, especially if your CEO(s) is strong. Simply warn them that a bidding war is in order. If it is your state corps, you have a couple of options; let them buy them as Dragoon said (but be mindful that they are leaving them in your country), or nationalize your state corps. Blocking CEO's altogether can be both good and bad....it is according to how much experience you have....if you are new, I would not advise it.