If you are currently a Trusted Person on someone else's Crumb account, and you would no longer like to be, the ideal course of action would be to request that the owner of that account remove you themself.
If this is not an option for whatever reason, you can remove yourself from their account's list of Trusted Persons. Here's how to do that:
Open Owner information, and click on Trusted Persons
You can open 'Owner information' by clicking the small circular icon in the top right of the app. This will either be your profile picture, or your initials.
Scroll down to 'Accounts trusted on' and click on the account from which you'd like to be removed
This will be their name and email.
Click on 'Leave this account'
If you're sure you'd no longer like to be this person's Trusted Person, click the red button that says "Leave". You'll immediately be removed from their list of Trusted Persons.