I remember a while ago I heard of something that slowed reactions and was effectively the opposite of a catalyst. What is this called? Also would appreciate examples and an explanation of their uses just for curiosity. (I think the one I heard of was used to prevent spontaneous combustion)
The opposite of a catalyst is essentially an inhibitor. From IUPAC's gold book Gold book
inhibition: The decrease in rate of reaction brought about by the addition of a substance inhibitor), by virtue of its effect on the concentration of a reactant, catalyst or reaction intermediate. For example, molecular oxygen and p-benzoquinone can react as 'inhibitors' in many reactions involving radicals as intermediates by virtue of their ability to act as scavengers toward these radicals.
Then there is a term called negative catalyst, and there are various opinions as to how they work. Journal of Chemical Education