(Adjective) Used or devised to stop something from happening, or to stop people from doing a particular thing.
(Noun) Something that stops something unwanted from happening, especially something that protects against illness.
1) Done before others can act: Done before somebody else has had an opportunity to act so making his or her planned action pointless or impossible.
2) Intended to prevent attack: military intended to eliminate or lessen an enemy’s capacity to attack (a preemptive strike).
3) Discouraging further bidding
I hope this helps.