Hello,

Though it is not possible to remove the ability to re-assign tickets, you can setup an observer rule to revert the change made and also alert the agent that he/she wouldn't be able to carry out the action on his/her own.

Here's how you can configure the rule :

When any of these events occur : Agent is updated from Agent ABC to ANY

And the events are performed by : Agent -> Choose agent name

Actions : Assign ticket -> Event performing agent

Send email to agent -> Event performing agent

Cheers!