How can I restrict which computers can connect to an instance of SQL Server?
If you want to totally restrict certain servers, then the Windows Firewall should be configured to the restriction.

See: Configure the Windows Firewall to Allow SQL Server Access

If you need to prevent a specific login from connecting, then of course you would need to manage that login's rights to the server.

Depending on the granularity of what you need to do, you might need to use both approaches.