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.
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