Step 1:
To disable the use of Sofort for future transactions.

1) Navigate to Settings > Configure Chargebee > Payment gateways > Smart Routing.

2) And click the pen icon to remove the payment gateway associated to it. 

Step 2: 
To disable Sofort payment option from gateway. 

1) Navigate to Settings > Configure Chargebee > Payment gateways .

2) Disable to the Sofort option highlighted in the screenshot below from the gateway settings page.
Note : This action would disable the flow that handles Sofort payments.