One character has laid out 6 Smoke Cloud markers, there is an adjacent square next to it with clear terrain. two of the Smoke Cloud markers are placed diagonally, near this square.

If an opposing character uses Charge to get to this square via the diagonal access, they have reached (& passed through) hindering terrain and they stop.

Can they use the CLOSE: element or is it 'cancelled' out as movement was ceased by the hindering terrain?

You may perform your CLOSE, because it is move

Then CLOSE. You always have to finish the move before the CLISE attack. It is mostly the same situation as moving next to the target of your attack, which normally ends your move action

How’s that for a 2 part answer?

Cool, this was how we ruled it but one player felt quite adamant it meant no CLOSE. Easily done.

That player is probably confused by old hypersonic speed rules where the attack happened during the movement so if the movement stopped you could not attack. Now barring special powers all attacks happen after movement ends so it doesn't matter if they are forced to stop.