Depending on the process you live in your team it maybe required to allow users to close tickets, even if they are not having the lock or not being the owner of the ticket.
Be careful on this setting!
Since the ticket system is ensuring, that no concurrent operations are executed without having the lock on the current task. E.g. assume a client places a task and your colleague is processing the operations on the task. Now the client calls again and tells a different agent, that the placed task is obsolete. So the second agent simply closes the ticket, even if this ticket is currently being processed. The client will not be willing to pay for the effort, since he reported early that he do not need it anymore.
To modify the behavior you can simple disable this setting