The easiest way to remove your accounts is to log in to each one and follow the steps below:
Instagram:
Log in to your Instagram account and click on Edit Profile
Click on Manage Applications
There will be a section called Stackla, click on Revoke Access to remove the Instagram account from Stackla.
Facebook:
Log in to your Facebook page and click on Settings
Click on Apps (located on the left-hand side)
You will notice that a Stackla icon will appear at the top of the page
Hover over the icon and click on the 'x' to remove the access.
Note, if these options do not appear then your account has not been connected.
For further information please either send an email to support@stackla.com or log a support ticket.