Tell it no first and if it doesn't stay down then get a water bottle and squirt water at the cat when ever it tries to climb the counter.
Squirting water at an animal may sound mean but it doesn't hurt them and works to correct behavior.
There's also a device on the internet that you can place on your counter that will keep the cat down.
http://store.petsafe.net/ssscat-spray-deterrent
I haven't used it so can't say anything about it as I don't have any pets. But it's what I found when looking up other ways of keeping cats down from counters.