Inhibitors
There are substances that lower the rate of a reaction just as others raise it.
Inhibitors, or retarders
Substances that lower the rate of reaction are called inhibitors, or retarders.
An example
Adding a drop of sulphuric acid in order to lower the rate at which hydrogen peroxide decomposes.
The very same substance — hydrogen peroxide — decomposes quickly when manganese dioxide is added and slowly when a drop of sulphuric acid is added. So the rate can be steered in either direction.
Why this matters in practice
A fast reaction is not always what is wanted. Preserving food, protecting metals from rust and storing medicines are all cases where slowing a reaction down is the whole aim.