Web Filter provides a useful, and often overlooked, tool to help restrict access to personal Google accounts while still being able to utilize your company's Google accounts and applications.
Navigate to Apps > Web Filter > Advanced. The option is called "Restrict Google applications" and is disabled by default. The image below shows the option when it is enabled.
Besides enabling the option itself, you also need to specify the Google-hosted domains you want to allow access to. Any domains not listed will be blocked by the Web Filter.
The other important item to note here is the warning about the SSL Inspector. for this feature to work you absolutely must be using the SSL Inspector on your network. Without it the Web Filter cannot filter this traffic.