When on trial, you simply need to go to the top left corner - Administration - General - Delete organization.

(and solve a math question to avoid unfortunate mistakes)

First, you need to cancel your plan by going to the top left corner - Administration - Billing - Select plan - Cancel subscription.

(this can only be done by the administrators of the organization)

Second, you will be sent to a canceled organization screen, where you can choose to select a new plan. In this screen, press on the top right corner - Administration - Delete organization.

(and solve a math question to avoid unfortunate mistakes)

Have you signed up with the wrong email address and you need to delete an old account? Simply go to the top left corner - My profile - Delete account.

(and solve a math question to avoid unfortunate mistakes)

Please note that you will lose action to the account and all its data. Cards that you have been assigned to and are shared between others will not be deleted.

There is also one restriction to deleting accounts. If you are a non-administrator member of at least one organization, you cannot delete your account. In this scenario, you need to ask the admins of said organizations to first remove you from those.