To cancel your ServiceM8 account you must be the account owner and it can only be done when you login your account online (i.e. through ServiceM8's web interface on a Mac or PC, not the ServiceM8 app) at https://www.servicem8.com/login-page.
In the Dashboard, Click Settings
Click ServiceM8 Account
Click Cancel Account and follow the prompts.
Note: Once you cancel your account all the data will be removed and is not recoverable. You must follow the cancellation wizard all the way to the end then log out of the account to finalise account cancellation.