Maybe something like "Moderators, at their discretion, may remove comments that can be construed as advocating illegal activity" would work for a formal policy - it reads like relatively inoffensive boilerplate and would be something to point to if a post like mine needs to go, but is vague enough that it doesn't scream "CENSORSHIP!!!" to people who feel strongly about it. The "at their discretion" is key; it doesn't create a category of posts that moderators are required to delete, so it can't be used by non-moderators as a weapon to stifle otherwise productive discussion. (If you don't trust the discretion of the moderators, that's not a problem that can be easily solved with a few written policies.)