poison pill

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A strategy intended to make a hostile takeover of a corporation more difficult, as by granting of special rights to existing shareholders upon the occurrence of the purchase of a significant amount of stock by an intended acquirer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun business Any strategy designed to produce negative results for an entity carrying out a takeover.
• noun politics A provision in a bill that leads to people who would have supported it opposing it instead

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the target company defends itself by making its stock less attractive to an acquirer