How to avoid propogation of changes

[Chuck . Irvine]

At any given time, we have multiple releases of our software under 
development. Each release has it's own branch, with the latest release 
being developed on the trunk. We propogate changes downstream by doing 
merges from each release branch to the immediate downstream release 
branch. 

Here is the problem: What if we make a change that we don't want 
propogated downstream. Does any one know of a good way to accomplish 
this? 

Thanks

Chuck