There is a closed post related to this but it still doesn't seem to be resolved: https://idealab.foundant.com/forums/597718-communitysuite-idea-lab/suggestions/46455940-inactivate-a-letter-template

I'd like to be able to hide, delete, or deactivate a grant letter template even if it has been used with a check. When used with a check, you cannot delete a template. This would help clean up the list of available grant letter templates to choose from when some become defunct.