I’ve already submitted payment but it was $0. But i decided to go with another tax preparer at the last minute. How can i cancel this filing?

If you have not e-filed the tax return by clicking on the  button labeled "Transmit my returns now" in the File section of the program on Step 3 then you can just abandon your TurboTax account and file somewhere else.

Otherwise you have e-filed your tax return and it cannot be retrieved.  You will have to wait for the IRS to either accept or reject your tax return.  If rejected you can then abandon the TurboTax account and file somewhere else.  If Accepted the IRS has your tax return and will process the return as filed.  If you use another provider and try to e-file your federal tax return again it will be rejected by the IRS.