If I stop getty for the following vt I can achieve what I want, but only until reboot
systemctl stop serial-getty@ttymxc1
1) How do I make it permanent?
"disable" does not help.
2) How do I make it irreversible?
If user takes sdcard out of device and fiddles with insides, he still shall not be able to restore access.
P.S. If it's possible I don't want to touch uboot.
A few more useful points from discussion in comments:
Breaking pins is not something I may decide, this is a mass production hardware and handling is done by hardware team. I'm sure this stuff is already long before decided, schematics won't be changed in a last minute if it's not something terrible.
In 2) I asked for solution to be irreversible. The reason is that, customer wants no access to console possible, even if soldering is performed. Technically, solution like
systemctl stop serial-getty@ttymxc1 is easily reversible, so I'm interested in if there is any other more bulletproof possible ways.