You may set up a custom answering rule with a call condition with a Specific Date Range, which allows you to designate the time when the custom rule begins and ends. You can either send the calls to voicemail directly or play a greeting and disconnect. You may check the KB article below on setting up a custom answering rule for a user.

https://support.ringcentral.com/article/How-to-access-the-advanced-call-handling-feature.html