Is it OK to decline a review request after multiple resubmissions?
That's a tough situation and not one I've run into, so I can't speak from experience.  In your place I think I'd decline to review, and directly contact the editor to explain your position. (If the review request doesn't have an explanatory text box associated with it, then email the editor directly.)  Something like "The authors have been cooperative but the paper quality has remained low for multiple revisions, and at this point I have no more constructive comments to give on this paper.  Unfortunately, my time doesn't permit another review" seems fair and clear enough.