I did that too initially. In my case, (probably yours too) truecaller is the reason why I get so many spam calls.

Ever since I installed it they increased from one per week to 5-10 a day. I have evidence that truecaller has sold my contact to call centers. I created an email and registered truecaller with it and used it for literally nothing else. I started getting spam shortly after.

And the number that I didn't give truecaller doesn't even get one spam call.

Changing your display name doesn't do anything. They have your real name from your social graph.

The only way to hurt their business is to unlist your number. If lots of people do that it'll become useless and more people will uninstall it.

Btw you can report spam calls to telecom regulators (govt). That's when I stopped getting them permanently.