Cease. Halt. Desist.
When you just need someone to give you a darn break, smettila (pronounced “zmett-i-la”) is the phrase for you. It means, roughly, 'stop that!', 'quit it!' or 'knock it off!'
It's a form of the verb smettere, 'to stop' or 'to quit'. You can use it to mean stopping something for good – like smettere di fumare, 'to quit smoking' – or just for a while (like smettere di piovere, 'to stop raining').
But when you turn it into a command (“smetti!“), it's quite clear you're talking about stopping that right now.
The la that you tack onto the end means 'it' or 'that', and it might refer to something specific, but equally it might not. For instance you often still include it even when you go on to spell out what you want someone to stop doing, where in English we'd drop it.
Smettila di urlare!
Smettila con quel tono arrogante!
Drop that arrogant tone!
Don't forget that you'll need to re-conjugate the verb if you're talking to more than one person: it becomes smettetela if you're telling multiple other people to stop, smettiamola if you're inviting a group to stop along with you, or la smetta in the unlikely event you're so bold as to give orders to someone with whom you use the formal Lei.
Smettiamola di litigare!
Let's stop arguing!
Eh ragazzi, smettetela!
Hey guys, knock it off!
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