Hi Jeroen,
Sorry for the delay, we have been busy with the release of another product.
In the latest version of WildFire, all the preferences are stored in the user's personal address book. The easiest way to see/delete them is to open "Contacts", then "Advanced" in the left column. Finally you should see a list of your accounts.
The account "WildFire Preferences" contains all the major settings, including the list of groups. Each account you have created will also be shown here, and also contain the names of any groups that account is in.
Your safest bet may be to remove all accounts, and the Preferences.
Regards,
Andrew