If your employees would rather have your competitors product then you need to get to work rather than to forbid them to use it you need to make it so they would prefer to use their own.
Back then, Ericsson (And later, Sony Ericsson) was working really hard to make great phones, as well, and they certainly did. I never felt like I was lacking with my Ericsson phones even if it wasn't the same as everyone else's Nokia 3310.
Just like it's hard to make the new Facebook, it was hard to beat Nokia. Remember, back then, everyone was using Nokia 3310. It didn't just take a better product to become the thing everyone wanted to have, because everyone didn't want a mobile phone. They wanted a Nokia mobile phone.
If your employees would rather have your competitors product then you need to get to work rather than to forbid them to use it you need to make it so they would prefer to use their own.