How to stabilise the bicycle seatpost and saddle when it is all the way up?
Your seat post is not all the way up, it's too far up.

If you need the saddle at that height you need a longer seat post. As mentioned in comments there is usually a 'minumum insertion' point marked on the post.

Don't ride the bike with the seat post too far out of the seat tube. It's dangerous as the seat post may collapse under you and you risk damaging the top of the seat tube even is that does not happen.