Hello there. I was struggling with this issue but I have found a great fix. The AG target undesignate key combo is LWin - N. I couldn't get this work from the keyboard let alone via my X52 Hotas mapped key. To fix this, I went into the Controls section in the Harrier configuration, deleted LWin - N for the keyboard and inserted a mouse button on my X52 directly in DCS. Now the Undesignate function works like a charm. I normally try to not program my Hotas in DCS, but use the X52 Hotas programming software instead, in order to get the 3 modes to work and the Pinkie functions. Doesn't always work though, as this example demonstrates. Looks like this is still a bug, but my solution works well.