If you wish to disconnect from a planner as a participant, you can do this from the My Connections page (found under Connections).
This page will show a list of your connections, their phone numbers, and their Global Permissions (Full Access or Custom). You can see more, and edit the connection with the Edit button on the right-hand side of each row. Clicking this button will show you the Connection Details page. From here you can see their Connection Information (Name, Email and Number) and edit their Permissions. To Disconnect from a planner, simply click the Disconnect button, and then confirm with OK. Depending on the set permissions, you may have to scroll down the page to get to the Disconnect button.