I would like to know if it is possible to disable the person who created the pull request from reviewing and authorizing the request on Azure DevOps Server 2019?
Right now everyone can review and authorize their own pull request and we would like to avoid this.
Yes, you can. Go to branch policies and first enable the policy Minimum number of reviewers. Next make sure "Allow requestors to approve their own changes" is unchecked. You can do so under branch policies:
The screenshot is from Azure DevOps, but it's also available for server, as you can see in the documentation at Improve code quality with branch policies - Require a minimum number of reviewers.
If Requestors can approve their own changes isn't selected, the creator of the pull request can still vote Approve on their pull request, but their vote won't count toward the Minimum number of reviewers.