Linux Mint / Ubuntu offers a simple tool through which you can block access to specific domains or websites completely.
Here is how :
1. Go to Menu > Administration > Domain Blocker. (For the Terminal fans, type sudo mintnanny to bring the window shown in step 2).
2. Click the Add button and type in the domain name to which access needs to be blocked, once done click ok.
Try loading the website address or the domain name that is blocked to confirm the changes.