re: "How to exclude certain users from seeing the JOB?"
Upwork does not have an option to post a public job and exclude certain users from seeing it.
You CAN post a job as a private job, and only invite certain people to that job.
You can post a public job, and it will potentially be seen by everybody, but if there are people you don't want contacting you, you can BLOCK them using the Upwork messaging tool. You can block anybody who you have had previous contact with, and completely prevent them from messaging you again through Upwork.