As long as you have the sender's email it's a simple procedure to block someone on Gmail. And the process is the same for the mobile apps and Gmail website on a PC browser.
Procedure to Block any Email-
1. Go to Gmail
2. Enter your credentials for signing in.
3. Open the email you want to block.
4. Click on the three-dot menu button at the top-right corner of the email and hit the block button.