Distract. At 18 months he will have absolutely no clue about social conventions and cultural issues around staring and manners. At three or four, you can say 'no staring, darling', and then distract them, but at 18 mos? Distract, distract, distract. He isn't staring to be rude, he's staring because he wants to learn about the world and everything is fascinating. It's a lot more sensible than the social niceties that he'll get brainwashed into later. He's just being naturally inquisitive.
If he's chucking stuff because he's done, just say 'no, thank you, x. No throwing' calmly, and put the cup back. Ask him if he's finished. Or if he would like more? Even if he is non verbal as yet, you can be giving him basic choices and allowing him to communicate. (The throwing is of course a communication in and of itself - you just need to get him to channel that communication a bit more effectively. For example - you need to notice he has finished his drink before he lobs it, and ask if he has finished, or if he would like some more? Pref with your hand on the beaker.)
then just move it out of reach. No lobbing involved.
If you pre-empt the action that you know is coming, he'll pretty soon forget it and learn a more appropriate method of ending his meal. The lobbing stage doesn't last long. It's even shorter if you get to the plate/ beaker/ hand and remove it from the equation. Distraction is your best bet.
I didn't ever give mine random food during the day. They had three meals, and they had two snacks. Snack time was always seated, not during play, at this age. Finding half chewed cookies in the duplo isn't my idea of a good time. Later on, when they are old enough to understand rules (and not so liable to stand on it/ choke on it) then fine, whatever. That way they know that the table/ highchair is for eating, the floor/ playroom (or indeed the table with no food on it) is for playing.
Because there was such a short time between meals / snacks, I didn't ever worry that they were going to be hungry. If he lobs his food and you take it away, it's only an hour and a half until snack. He won't die of starvation.