If you wish to remove a portfolio, you can do so directly from your App. To start, go to your "Holdings" tab on the top of your screen.
Once you are on the holdings page, you can go to the Portfolio Edit Screen by tapping the small icon in the upper right.
Once you are on the Edit Portfolio Screen, just tap on the three dots to the right of the portfolio you would wish to delete.
You would be given a selection of choices, one of which being "Delete Portfolio". Select this option and follow the prompts to remove the portfolio. This will work with both Manual and Synced portfolios.
If you are looking to cancel a Managed account, please see the article here for instructions.