put a cap on (something)

To establish a clear upper limit for something. If we don't put a cap on rent in the city, people are going to start being put on the streets because they won't be able to afford anywhere to live. I stick to a strict budget, putting a sensible cap on how much I can spend each on non-essential things.

put a cap on something

Fig. to put a limit on something. We need to put a cap on spending in every department. The city put a cap on the amount each landlord could charge.