The phrase "give the green light to" means to give approval or permission for something to proceed. Antonyms for this phrase could include "veto," which means to reject or forbid, "prohibit," which means to disallow or ban, "obstruct," which means to impede or block, and "hinder," which means to interfere with or constrain. Other antonyms could also include "disapprove," "condemn," "oppose," "denounce," "forbid," "negate," "restrain," "preclude," "prevent," or "thwart." These are all words that indicate a refusal to allow something to proceed or a lack of approval for a particular action.