You can use these settings to tweak the Auto-Close behaviour:
This one is more nebulous. 
The Trust Levels are more about how well you know the particular forum rules/mores, so a post should only be flagged by a TL3 Regular user if they think it’s against the forum rules - not just because they disagree with it, or don’t like the person that posted it. If you find a TL3 abusing their extra perks (eg. Flag weight) you can demote them back to a lower trust level (and even lock them there if you want to).
If you don’t want any TL3s at all you can also tweak the promotion thresholds to make it impossible to attain.
The Trust Levels are quite a big feature of Discourse though, so you might want to explore them a bit before you make any decisions (same for the flagging).