Keep in mind that with the changes to the edit review system, "Too Minor" was removed as a reject reason in mid 2014 because the definition of too minor was very inconsistent. So in that sense there is no longer an edit that is considered "too minor", and the responsibility for addressing is pushed to the reviewers to make a decision on how best to handle.
- Does the post even need fixing? And does the edit improve the post by making it more clear, easier to read, and/or fixing grammar/spelling? If not, then reject as "no improvement whatsoever"
- Does the edit fix everything? Is there anything left to fix? If not and you are willing to finish the edit yourself then you should either click "Improve" or "Reject and Edit". Don't just reject because you think someone is farming rep with minor edits. I'll refer back to #1 first (does the edit improve the post - if so then it is useful even if it isn't complete).
Now nothing in the above even remotely covers your actual question because your asking about what to do when a "too minor" edit is approved. But the reason I covered the background is it helps set the stage for what to do after it is approved. And the solution is literally restarting #1 above.
- Is the post easier to read (or find in the case of tag edits)? If
so, then leave it alone.
- Did the edit make the post harder to read? If so, then roll it back.
Now if you find someone approving a lot of bad edits, then your only recourse is to flag for a moderator and provide some specifics on the bad edits they were approving.