By mistake some user have posted manual JE to Retained Earnings account. We found the problem and reversed the JE. Now management wants us to restrict users using the RE account. SO i made changes to segment qualifier tab natual account definition value set to say Allow posting : NO. 1)what is the impact of changing to value to NO.
2)does it impact year end closing process or any other processes in GL.
3)Is thier any other method to restrict users posting to RE accounting.

You should change the allow posting attribute of account combintion to No. Changing this attribute just for a segment value is not going to do the job unless you run inheritance program.
Once you change the allow posting attribute of the retained earinings combination to No, it should work as expected.
It will not prevent Oracle from posting activities to retained earnings account.
You can consider using Security rules to prevent users from entering journals against retained earnings.
All changing should be done in the test environment and throughly tested before promoting them to production environment.