How to activate Contact Form 7 only on one website and not all the whole network ?

Greetings friends,

Quick question. I want to add a contact form on the main website of my WPMU blog network...

So I added the excellent "Contact Form 7" plugin (as well as "Contact Form 7 Honeypot")to my network and activated it.

Now the problem is, the plugin is activated on ALL child sites, and they ALL have access to the main form.

What I want to do is to activate the plugin JUST on the main site, but it shouldn't be activated for all child sites.

Is that possible, and how should I do that ?

Thanks a lot in advance for your help,