If an user can navigate to different websites in a new window, they will be able to do the same in the original window as well.
However, you may want to try the tip here: https://confluence.atlassian.com/display/CONFKB/How+to+Force+Links+to+Open+in+a+New+Window
This is for forcing links opening in new window, but if you replace _blank with _self I believe it should have the effect you are looking for.
Do you use templates with Confluence? Take part in a remote 1-hr workshop. You'll receive USD $100 for your time! We're looking for people to participate in a remote 1-hr workshop...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs