Go to the Ads Manager:
Select the campaign you want to stop.
At the top of the section, you may find a toggle switch; toggle that on (slide to the right) or off (slide to the left).
To turn an ad set on or off, select the ad set and toggle the switch.
To turn a specific ad, scroll down to ads, select the advertisement and slide the toggle switch.
You can delete an ad, ad campaign, and ad set from Ads Manager on the desktop.