How to discard change sets on component

Hi,

I have delivered some changes to remote stream.
Suppose the history is:

1.add a
2.add b
3.add c

I want to discard 1 and 2 change set, but leave 3.
I select 1 and 2 from history view, and right-click them, select 'Discard...', then I see the history has 3 only.
But from the 'Pending change' view, I find that there has an outgoing pending change from remote stream, what I can do is only 'accept', if I accept, the change set 1 and 2 will come back....
My question is how can I discard history from workspace and then deliver them to remote stream?

Many thanks!