What Slows Down Or Stops A Chemical Reaction?

Basically what you're looking for is the opposite of a catalyst, which is a blocker, inhibitor or an anti-catalyst.
Instead of speeding up a chemical reaction, it slows it down or all together stops it.
Chemical/biological inhibitors are generally found in the body in areas where you don't want a chemical reaction to occur. They are good for helping with allergies, etc.
Good luck!
:D

If an endothermic reaction, taking away heat will slow it down. In general reactions double their rate for each increase in temp by 10 degrees celsius (pretty sure, it has been a while and it is just a rule of thumb). Adding pressure will slow reactions that have a gas as a product. Changing the pH and the concentration of reactants also can slow reactions.

A retardant will slow down chemical reactions

Catalyst

A catalyst