Detecting when the battery is charged is simple. Create a profile with an event trigger "Power Full".
Now truning off the plug is the hard part. I suppose you can always have Tasker do a "say" command thet yells "Alexa, turn off charger plug" or something. Alexa does not really have a interface that will let you do this quietly.
My smart home system (Homeseer) accepts HTTP commands, so I just send the appropriate HTTP command to it. You will need to indicate what type of smart home system (besides Alexa) that you have.