* Deny: This is a direct opposite in the sense of refusing to give something. "He was allotted a room, but she was denied one."
* Withhold: This implies preventing someone from getting something. "They allotted funds for the project, but the board withheld approval."
* Deprive: This implies taking something away from someone, often forcibly. "They were allotted food, but then they were deprived of it."
* Reduce: If you allot a certain amount and reduce it, you are giving less. "They allotted 10 hours for the task, but then reduced it to 5."
* Take away: This is a general term that can be used in many situations. "They allotted him a week of vacation, but then they took it away."
The best opposite will depend on the specific situation and the intended meaning.