Hi.......I trimmed mine the other day, though it is only about 6ft tall. Starting at the trunk, just pull them gently, you will find the come off easily in rotation...if they are further up snip them off near the trunk with secateurs. I had to take a lot of mine off, so shaped the whole Cordyline like a fan...it looks quite good.....:o)

And if your secateurs are a bit blunt, a sharp pair of scissors is best.