##priests

If the priest caste is required to create the runes (and drive he rituals that power these runes), then they will be doing so because of the gods. If the gods decree that only warriors get the "go directly to heaven, do not pass Go, do not.." treatment, then the priests will know this better than anyone.

Priesthoods tend to be the source of long-standing traditions and religious laws. They have a vested (pun intended) interest in maintaining that status quo.

##the gods

If your gods are the power source for these rituals, then the gods themselves can define punishment(s) for those who break the law.

• the runes simply don't work if placed on a non-warrior (because the gods know...)
• the gods curse the non-warrior and/or the priest with disease, a disability of some sort, and/or a painful death
• perhaps they are cursed to become wraiths by committing such heresies!

##society

In addition, society itself may help enforce such rules. If a rich noble is seen sporting a rune he purchased illegally, it's possible the noble will be killed on the spot by a mob of angry peasants who fear the gods' retribution for such a sacrilege. (Though this begs the question of what happens to retired warriors...) Overzealous mobs have been known to do worse for lesser reasons throughout history.

##other warriors Perhaps other warriors consider it a sacred duty to carry out divine retribution against any who would dare commit such a sacrilege. They might first desecrate the rune (ouch!), then see that they suffer and die somewhere far away from the priesthood, ensuring that they never receive proper burial. This, then, dooms them to either becoming wraiths themselves or becoming a tasty snack for other wraiths.

And then perhaps they go after the priest who created it, too...

##the rune It is possible (depending on how you define your runes) that the rune is not the exact same artwork for everyone. Not just a "Everyone gets stamped with the same swirly pattern," but a unique mark. If that mark is unique for each priest who creates it then there's a signature of who did the deed.

That signature can then be traced back to the priest. And that priest can be killed, defrocked, excommunicated, disfigured, etc., for the crime of creating it illegally. This is a built-in incentive to not get caught.

priests

If the priest caste is required to create the runes (and drive he rituals that power these runes), then they will be doing so because of the gods. If the gods decree that only warriors get the "go directly to heaven, do not pass Go, do not.." treatment, then the priests will know this better than anyone.

Priesthoods tend to be the source of long-standing traditions and religious laws. They have a vested (pun intended) interest in maintaining that status quo.

the gods

If your gods are the power source for these rituals, then the gods themselves can define punishment(s) for those who break the law.

• the runes simply don't work if placed on a non-warrior (because the gods know...)
• the gods curse the non-warrior and/or the priest with disease, a disability of some sort, and/or a painful death
• perhaps they are cursed to become wraiths by committing such heresies!

society

In addition, society itself may help enforce such rules. If a rich noble is seen sporting a rune he purchased illegally, it's possible the noble will be killed on the spot by a mob of angry peasants who fear the gods' retribution for such a sacrilege. (Though this begs the question of what happens to retired warriors...) Overzealous mobs have been known to do worse for lesser reasons throughout history.

other warriors

Perhaps other warriors consider it a sacred duty to carry out divine retribution against any who would dare commit such a sacrilege. They might first desecrate the rune (ouch!), then see that they suffer and die somewhere far away from the priesthood, ensuring that they never receive proper burial. This, then, dooms them to either becoming wraiths themselves or becoming a tasty snack for other wraiths.

And then perhaps they go after the priest who created it, too...

the rune

It is possible (depending on how you define your runes) that the rune is not the exact same artwork for everyone. Not just a "Everyone gets stamped with the same swirly pattern," but a unique mark. If that mark is unique for each priest who creates it then there's a signature of who did the deed.

That signature can then be traced back to the priest. And that priest can be killed, defrocked, excommunicated, disfigured, etc., for the crime of creating it illegally. This is a built-in incentive to not get caught.