Keep in mind that toddlers have small tummies and sometimes need to snack. The trick is often to limit how much you offer for a snack and to limit milk drinking in between meals, as that can fill them up too.
We had a similar issue with sitting at the table when Tyler first decided he wanted to sit in a real chair. He just would not stay in there. What I did was clear his food away and put it on the counter as soon as he got up from the chair and say "oh, you're getting up, you must be all done." If he immediately came back and sat down, I'd give it back and let him eat, but if he got up again, then mealtime was over and there would be no more eating until the next snacktime. There were a couple of cranky days, but it didn't take him long to figure out that when he got up, that meant he was done.