I just want to add that I have came across this need quite a few times when doing complex setups with order tags. I understand that you can put the item into a different product group, but imagine if you have a complex order tag group with many order tags and just have one tag you want excluded for a particular product. The only way to do with a different group is to clone the whole order tag group for that other item. Then if you make changes to the order tag group, you need to remember to either make changes on the other order tag group as well or do the clone again.
Yea… I was thinking about this… I have currently 21 pizza flavors, that can be combined with each other in up to three flavor per pizza… that gives me a lot of pizza combinations… LOL…
the problem is that for the moment I have only one pizza that cannot be combined because it already is a three flavors pizza…
I regulary do contests in wich clients try to invent a pizza combining flavors… the most voted wins, and its included in our menu… so in the future I will have more pizzas that cannot be combined…
Putting them into a different group as @Jesse suggested seems at the moment the best (and cleaner) solution…
I imagine having 10 pizzas that cannot be combined… and having to add them into the mappings as denials… not nice… LOL