The word "decriminalize" means to remove criminal penalties from an activity that was previously deemed illegal. Therefore, the antonyms for this word would relate to making something illegal or imposing criminal penalties. Some potential antonyms for "decriminalize" include criminalize, sanction, prohibit, condemn, convict, punish, or prosecute. These terms indicate that the activity in question is considered illegal and subject to punishment under the law. While "decriminalize" implies a lenient approach to a particular activity, its antonyms imply a more stringent attitude and harsh consequences for those who engage in the activity.