I suggest you look at https://www.ibm.com/docs/en/elm/7.0.2?topic=tool-configuring-your-project-area-code-review. Presumably you just perform the reverse steps to deactivate it.

Unfortunately there is no official way to deactivate using the Code Review tool, while at the same time showing the Code Review presentation to allow the normal workflow of opening up the Code Review tool.

However there is still a workaround that might work reasonably well.
As you and David Honey pointed out, you can remove the Code Review presentation part from the work item. This Code Review presentation part contains the UI for both creating (initially submitting) the code review, as well as the hyperlink for navigating to the Code Review tool itself once one exists. However I believe we added a link from the work item to the code review itself. Thus even with the code review presentation removed, if you navigate to the Links tab in the Work Item editor, you should still see a link to the code review object (if one had been created in the past), allowing you to easily navigate to it in the web-ui. If you decide to go with this approach, then you just need to educate your users that you removed the presentation, but the Links tab can allow access to old code reviews. I have not tried this, but I believe once in the Code Review web-ui, you can still do everything as normal, even though the presentation was removed from the work item (but you can try this out and let us know if there's any difficulties with this approach).