Does a Pull request can be locked? or can raise pull request using a specific commit id?

[vinoth SS]

Currently we have a branching model like below

feature/A --> dev --> master

 When u create a pull request from dev - master and wait for the review , in mean time some one merge a newcode in dev is automatically gettign updated as part of existing pull request from dev-master. Do we have an option of locking that? or is there any restriction not any new update gets added to the pull request.