The only proof they will accept is if one of their own fly to moon, survive the deadly sea of radiation and affirm the presence of the hardware.
I doubt any of the leftover Hoaxpreachers would ever admit that they were wrong for years.
Pick one or more of the following
[ ] the one who made the trip was bribed or threatened to lie (as you already noted)
[ ] the one who made the trip was hypnotized or fooled in some other way (can even be used by the one who made the trip)
[ ] it works now but not then
[ ] the relics were brought were later
[ ] the NASA was using secret technology (Area51 ... UFO....)