This is not unique to Mac OS. Windows 7 'pinned' applications, and Ubuntu app bar both have this same problem. While technically it would be nice to have a 'Cancel Launch' choice if you right clicked, the practical issue is that terminating a program mid launch is dangerous; the programmer who wrote the application is very unlikely to expect you to change your mind like that. Half-initialized data structures could be left dangling. Cleanly handling an aborted launch is heavily on the application developers side, and the OS developer can't tell which apps are safe to terminate early and which are not. Due to this, it's unlikely to see it anytime soon.
So it's not technically feasible to interrupt a launch in progress. But there is one way I've seen made popular by tablet interfaces: if a user clicks near a border, ignore the click. In other words, make the click targets smaller, closer to the middle of the icons, and borders become a 4-10 pixel 'no man's land' where clicks are ignored. This greatly helps in inaccurate touch devices, but it can easily be adapted to this use case as well.