Disabling Forgotten your Password Jira Service Management Link
Users are able to use the 'Forgotten your password?' link in the Jira Service Management login screen as below:
If this link isn't appearing and you're wondering why, please refer to Forgotten your Password Link Does not Appear on Jira Service Management Login Page.
Enable External user management in Configuring JIRA Options. The link will continue to appear, however users will be prompted to contact the administrators of the instance rather than being able to reset the password.
Note that, at the same page "General Configuration" Jira mode should be set to Private mode as well, as you cannot select Public Mode if External User Management is enabled.