A guide to implementing a mailing list off-topic tagging policy for moderators:

Step 1: ask your mailing list/group members to use the #OT hashtag on off-topic posts.

Step 2: watch as your mailing list/group members find increasingly inventive ways to type it: with dots, without dots, with and without spaces, in upper, lower and mixed case, with and without the hash symbol...

Step 3: realise that the only consistent guarantee is that it won't be used on off-topic posts, and give up.