luca, bring up the folder's context menu, select Properties. On the 'General' page you will find the option 'scan for new mail' - you want to turn this off for folders that you don't want included.

Here's a hint, if you want to turn this off for most of your folders, then select it from the properties of the root folder and use the 'apply to subfolders' checkbox. Then turn it on again for those individual folders that you actually want to be scanned