How can I hide some contacts from dialer?
I can't speak specifically to the Defy, but if you've got the stock Android Contacts app and you're syncing to Google, you can tell the phone which groups to display.

From the Contacts app Menu | Display options | {account}. Then check the groups you want to see, and the uncheck the ones you don't. "My Contacts" is going to be pretty much everything under your account. If memory serves, this is only for display; the contact info is still on the phone.

Please note that in Android versions 5.x and up this may be no longer valid due to a bug.