Fluorine: the most reactive element in our periodic table with the highest electronegativity, it steals electrons and such from all compounds and reactions, this can be used to stop reactions between what ever.
If it is mixed with Hydrogen you get Hydrofluoric Acid, which is very potent, more potent than Hydrochloric Acid because of its reactivity.
Argon/Other Inert Gas: it can be used to keep things from reacting because it is inert, it can be used in an environment, stopping anything from reacting
How many times does anyone have to explain that The Powder Toy is NOT going to become flooded with elements from the periodic table. I may lock this thread.