You can't, free capitalism at its finest( obviously I support free capitalism that wasn't supposed to be sarcastic )The only way you can stop a foreign bid is to bid higher yourself with another enterprise or country, if the corporation is in one of your countries. If you think they'll end up getting the corporation cancel contracts and take out all the money you can from it. You'll still get more out of it if they take it -upgrades since they pay extra when buying player corporations.