You can disable the Plesk upgrades only by modifying license keys, through the Partner API.
To disable Plesk upgrades:
partner10.upgradeKey with the API identifier "DISABLE_FEATURE_UPGRADES" in the "array of identifiers of upgrade plans" parameter.
For details on the methods specification, please see
http://central.plesk.com/docs/partner-api/1.0, section Specifications of Methods / partner10.createKey
Important: Once you have turned off a feature through the Partner API, there are no documented methods to turn it back on. If you need to perform this reverse operation, please use the web-based user interface, or contact your sales representative.