Yes, we can enable only the signup or only the check-in button, in the add new volunteer pop up.
The current build of V4S Mobile does not support the enabling of the signup or only the check-in button from the frontend. We at dftly can enable or disable the signup or the check-in buttons for your org from the backend. If there is any requirement please talk to the Salesforce Administrator at your org and they can inform us about the requirements.
Option to enable or disable the signup or the check-in buttons using the V4S Mobile configuration screen will be given in the future releases of V4S Mobile applications.