1) At the point of posting, if detected as less than 5 words, let the post go through, but let it not count towards the user's post count.
2) When you want to delete all the plus 1 type posts, the list is already stored in the forum DB (from step 1). Delete them as part of your cleanup. No users will therefore see their post count reduce.
People who want to post small posts (like only images or tweets) will therefore be free to do so. If they want it to count towards their post count, they can add 5 words in it. Completely their choice.
comment:
p_commentcount