I have an app on Heroku which is being used by few users. However, I notice there are some data issues which i\'d like to fix and stop the app in the mean time so users don\
Go to your dashboard on heroku. Select the app. There is a dynos section. Just pull the sliders for the dynos down, (a decrease in dynos is to the left), to the number of dynos you want to be running. The slider goes to 0. Then save your changes. Boom.
According to the comment below: there is a pencil icon that needs to be clicked to accomplish this. I have not checked - but am putting it here in case it helps.