Hi. I have a same problem. But I thing that that kind of functionality is correct and realistic. When someone is buying your products you have a contract, when you decide not to work with the company any more you are braking that contract.
I'm solving this with default internal sale on (all my product are sold internally). If I decide to sell to the AI, then I build a separate werhouse (just a purchase and a sales unit) and I manually disable internal sale for that product. This also gives you an option to raise the price for product that are sold to AI. When you don't want to sell to the AI any more, you simply click "stop purchase" button on that product. AI will soon discover there is no more supply and will start selling different products.
Hope this helps.
Regards MS

Thanks guys for your posts, please read my post again.

I asked if anyone else thinks there should be a button in addition to the internal sale that will allow a player to stop supply to AI companies at factories and warehouses.
How do you get rid of the AI once you decide you do not want to sell to the AI any more?

Also there is an option in the option screen to have "internal sale" on on standard when placing a sales unit.

Boxster77 wrote:It seems to me there is something missing in the factories and warehouses. What do I do when I do not want to sell to the AI any more?

If I could go to a factory or a warehouse of mine and click clients, select an AI company purchasing my product, then select a stop supply button that would be awesome! As of now the only way I know is to raise the price and wait for them to drop off a couple months later or destroy the sales units and rebuild them both make me irritated.

Click on the item you are selling and then below on the button on "Internal Sale".

It seems to me there is something missing in the factories and warehouses. What do I do when I do not want to sell to the AI any more?

If I could go to a factory or a warehouse of mine and click clients, select an AI company purchasing my product, then select a stop supply button that would be awesome! As of now the only way I know is to raise the price and wait for them to drop off a couple months later or destroy the sales units and rebuild them both make me irritated.