The answer I have come up with is the Navigated event. The user control that contains the web browser has a private boolean that determines if navigation is allowed.
If it is (set through the user control's Navigate method) then the control can navigate to a new page. Once the page has loaded the boolean is set to false, meaning back / forward is disabled.
In my case this adds a bonus: Links cannot be clicked on. I don't want these loading in the browser control - I only want this to view pages selected in the grid.
However, right-click on a link still has the open in a new window option.
This may not be the best solution, but it works for me.