I had the same problem on my Biancci mt bike, no mater what I did it would slip down.

I took it in to the shop and they replaced the seat post, have not had the problem since.

The odd thing is that the post that was on it was original to the bike. I don't know why it would not fit, but the new one fixed the problem. So, I would suggest if this is a new bike you go back to the bike shop and talk to them.

If you do decide to put the hose clamp on please make note that they put tape under it, to protect the post.

BarbaraAlys