We have a fairly new, but out of warranty, bathtub that has one of those
stoppers which are operated by the press of the foot.

Engaging the stopper is easy enough, but disengaging is all but impossible.
We now have to manually pull the stopper up, then wedge a blade into it to
keep it open while it drains.

We'd be quite happy to use an old-fashioned rubber stopper, but we don't
know how to remove the foot-operated device.

Any suggestions welcome.