Link to mcportalspawnevent
The event is cancelable.
If the event is canceled, the portal won't be spawned
The event does not have a result.
Importing the class
Link to importing-the-class
It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.
Link to extending-mcblockevent