Connect to the RingCentral API using PowerShell to search for and disable a user.

Save this file somewhere and call it within a PowerShell console.

Since this is likely something that would be used programmatically the username and password would need to be made available to the script as needed.
To accomplish this securely create an AES key using powershell, store it somewhere secure that only the script or trusted users can access.
Create a password with this key somewhere else.
Then enter the paths where indicated in the production section.

The credentials for your sandbox are not treated as securely and can be entered as plain text.
Though you can use the same key method if desired by using the same process.

To use with your production app:

Disable-RcUser -UserEmail someone@domain.com

To use with your sandbox app:

Disable-RcUser -UserEmail someone@domain.com -Sandbox