gummilutt, have you tried the Great Divide fences that ScaryRob suggested (post 17#). or locking the patio doors as Gremily_ and I suggested? It might not work (your Sims might just whine that they can't get through the locked doors), but both solutions seem a bit less drastic than removing the chairs. If you do have to remove them, I'd put them in inventory, so they're easy to get at.

OT: gummilutt, I noticed you have a diagonally positioned recliner on that upstairs patio. Are your Sims able to use that? Every time I place one of those on the diagonal, my Sims can't use it.

Strange indeed.. I'm going for the idea that for stupid sims, nearest thing to the left hand plate is all the way upstairs.
Another mod fix is Simlogical's "eating and serving controller", which allows you to specify where sims will eat at different times of day. It's a small vase for the table,

Andrew, I did, and posted results in post 20. Answer is no, it does not work. Locking door will prevent them from using the balcony, but they will still go upstairs and whine about the door being locked, so I don't see that as a better solution than just removing the chairs when I'm not using them.

ScaryRob, the recliner doesn't work, no. It's purely decorative, feigning the idea that the Sims sunbathe there at times.

Diovan, that sounds interesting. I'll look it up.

If I don't want the sims to use a dining seating area I turn the chairs backwards.

@Diovanlestat or anyone else that uses the eating/serving controller. Does the settings of the vase apply only to the room the vase is placed in? It says in description it can be used on a room to room basis, and there are no options on it related to room, so I'm guessing that's how it works.

It conflicts with Cyjons no table platters (that's why my Sims weren't placing the meal on the counter, Rob, I forgot I had a mod that prevents it), so I'm not sure if I want to use it. I wouldn't want a vase in every room of every Sim house just to fix that one particular house.

Sorry Gummilutt, just recently downloaded and unused, unfortunately my sims are too poor to have two tables.... their default eating position is standing up by the fridge door.
It reports as safe for Apartment life, and I'm guessing it is controllable on a room basis. I personally was hopping to use it in occasional rooms, like your balcony one. More of a switch to keep sims from eating in certain rooms, and allowing the main room to therefore be vase free.

Heads up for the conflict info, thank you.

The left turn information is interesting. I had a problem in the Ramirez' house. They would serve one plate on the end of a table in their kitchen, which was against the wall, and the rest in their dining room next door. :S