How do I disable the internal flash when using an external speedlite wirelessly on the 60D?

Hi,

I'm new here, but have come for help to a problem I'm having.

I own a 60D and a Speedlite 600 EX-RT.  No problem using them connected.

I've also managed to connect wirelessly to the Speedlite and use the built-in flash as the slave and the Speedlite as the master.

According to the user manual and other sources, I should be easily able to disable the internal flash from flashing and only using the wirelessly connected Speedlite.

Unfortunately, no matter what I do in the settings, I can only ever either fire both, or neither of the flashes.

It's a little frustrating.  I'm not totally new to photography or even SLR cameras, but it was many years ago (before the digital photography age) that I was really fit.  Now I feel I'm learning totally from scratch - fun but a bit slow.

Maybe someone out there has had the same situation and can help me, or point me in the right direction? Any suggestions would be greatly appreciated!