There are two answers to that, one general and one for the specific problem you mentioned.
How do you deal with conflicting bubbles or chains of trust?
You choose who you trust and then go from there. If you trust them, you should trust anyone they trust, thus forming the chain.
Say the trolls all endorse each other and claim you’re the troll, while you and your friends do the opposite. Who wins?
To even have a say in whether someone is censured or not you first need to be guaranteed by someone. My instance for example is guaranteed by fediseer.com (which is the root node to get the chain started, it won’t be used in the future because instances will guarantee for other instances). And for example kbin.social is guaranteed by me.
Each node in that guarantee chain has a say in who they trust and who they don’t. So, in theory, if utter.online starts spamming, discuss.online should withdraw their guarantee. If they don’t lemmy.dbzer0.com should withdraw their guarantee for discuss.online which in turn will invalidate its guarantee for utter.online.
How to get started with sharing my opinion on who to trust?
If you are an instance admin, find someone who will guarantee for you
Edit: Here’s a detail page for the instance you’re on.