When I encounter arm resistance of that sort in kotegaeshi, I find it's much easier to just change the technique, perhaps to something like rokkyo (hijikime osae, wakigatame).
True, that's a swift solution, but not necessarily the one that teaches the most. Some resistance problems are better to work on.
But if uke resists with great force exactly the technique you are supposed to do, then changing quickly to another technique is the aiki way to handle it.
One has to do it carefully, though, because uke will be way out of balance and probably not able to control his or her ukemi properly.