Symptoms
A login page is displayed allowing users to choose from security token services, specifically Azure AD
Diagnoses
When setting up the K2 for sharepoint app in Sharepoint Online, the Azure AD STS is automatically added to the list of claims issuers and by default is enabled for "UseForLogin"
Resolution
Using the management smartforms located in designer to update the claimIssuer values. This form is found at System -> Management -> Security -> Forms -> Manage Issuers
We set the "UseForLogin" property to false for the unwanted STS.