I would suggest ejectable seats. The table could include an electronic display, with a time count down: "XXX seconds for ejection". The last 60 seconds could have a sound alarm, so that more distracted custumers had the time to get out or maybe insrt a few coins for a few more minutes.
Just like we don't like to be treated as potential criminals when publishers put DRM on their ebooks, I don't like to be labeled as potential table hogger just because I pull out a Kindle while eating my lunch.
I know civilty and courtesy are rarer these days, but there should be another solution than just "no computers". Maybe the waitress could gently ask if they have anything else to order, and if not, ask gently to liberate the table for other customers.