Not sure what else to do though . They get told off but still ignore me
I wouldn't tolerate that. You are their Parent. Act like one.
House Rule is any food, snacks as well as meals is only to be eaten whilst child/children sit at the table.
All food they can help themselves to is put out of their reach and you give it out at appropriate times when they sit at the table.
Any food seen being eaten anywhere else is immediately confiscated and binned.
Ignore screaming, protests and tantrums and repeat The Rule.
Would they be allowed to do that in school, a restaurant or someone else's house?
Clue, any visiting child trying to cut up a chocolate brownie with a comb on one of my bedroom carpets would be swiftly put in their place.
Seriously, if you need parenting classes, or Supernanny please go and ask, you cannot carry on letting them do whatever they like and make excuses for them and for yourself.