The word "re-activate" implies the act of making something active again or resuming a previous process. Its opposite or antonym, therefore, would refer to slowing down or stopping that process altogether. Some possible antonyms for "re-activate" might include "deactivate," "disable," "turn-off," "shut-down," "halt," "cease," or "dismantle." These words can be employed in various contexts, such as technology, machinery, or biological systems. For instance, someone might need to deactivate a computer program that has a virus, a mechanic may need to dismantle an engine, or a doctor may need to stop or slow down a particular bodily function.