Hi silk, my commiserations on having to deal with a borked recycle bin!
You were close. The syntax of the ignore command is: dpcmd ignore-poolpart pooldriveletterwithcolon poolpartuid
The syntax of the unignore command is: dpcmd unignore-poolpart poolpartpath
For example, in your case above (assuming T is the letter of your pool drive - not your poolpart disk), to ignore that particular poolpart you would use:
dpcmd ignore-poolpart T: 422717a2-17f9-4c2f-b66b-1227cf2442c0
While to reverse that, unignoring it, you could use:
dpcmd unignore-poolpart \\?\Volume{c29fdf21-0f4b-41b9-9a1a-c540f54e1c28}\PoolPart.422717a2-17f9-4c2f-b66b-1227cf2442c0
(as for why DrivePool put the poolpart disk back into the pool after it was removed while missing, that's because when a missing disk is removed DrivePool can't make any changes to that disk to mark that poolpart as no longer in use - and normally the expected reason to Remove a Missing disk is because the disk is dead and won't be showing back up!)