Referee recommends paper rejection with no further comment - how to react?
Submit somewhere else.

The accept/reject decision is made by the editor, based on the recommendations of the referee(s).  In this case, the editor felt that the referee's opinion of unsuitability, even without any explanation, was sufficient to reject.  Maybe the editor felt that the unsuitability was self-evident upon inspection.  Maybe the editor trusts the referee's judgment so implicitly that they didn't feel the need to ask for any explanation.  Maybe there were other referee reports that the editor decided not to share with you.  We don't really know.  But in any case, whether this decision was made well or poorly, I don't think there is anything you could say that would be likely to change the editor's mind.

You could of course decide not to submit to that journal any more, and you could share your experience with colleagues.  You could also try to have your paper looked at informally by colleagues in the field, to see if it has severe flaws that you haven't noticed.  You could even complain to the editor-in-chief that you don't think the editor is giving due consideration to the papers they handle (though this if the editor-in-chief agrees that the paper is self-evidently unsuitable, this will only annoy them).

But don't try to pursue publishing this paper in this journal - it's a waste of time.