7 Taskbar Tweaker should do exactly what you want:
This programmes enables you to tweak your Windows 7 taskbar. The tweaks
are divided into three sections:
Taskbar items: control taskbar items related stuff, like middle/clic droiting, hovering, dropping, etc.
Grouping and combining: désactiver grouping, allow to drag windows entre groups, force/désactiver combining, etc.
Other taskbar options: souris wheel options, double/middle clicking sur le empty space du taskbar, and misc.
Réponse
To prevent seulement some applications from grouping dans le Taskbar, using Task Tweaker, follow the instructions dans le following capture d'écran. In this example, Je suis configuring it to prevent Outlook from grouping:
