Restrict the ability to assign to specific groups or users in the Support ticket interface based on the current user's organizations, ID, groups, or you can enable rules globally for all agents.
Install the app directly from the Zendesk Marketplace. Upon installation, you can configure the app with options and parameters. Hidden groups and users parameters will allow you to select the ID's of the users or groups that you would like removed from the assignee field in the Support UI. Targeted user/groups/orgs/user_tags fields allow you to specify the users that the hide-rules will apply to. If you would like the app to hide the hidden users/groups for all Support users, select the global option at the bottom of the configuration screen. For more complex rules, feel free to install more than one instance of the app.