How do I block users ability to email to a group in Exchange 2007

Do you want to restrict everyone to send mail to Group (apart from few users)?

- This I think you can set a option to accept  mails from only users are suppose to send mail to group on the group properties, mail delivery settings..

Or you want the group to hide from the GAL.
- If you want to hide group from GAL, then enable option Hide this object from GAL.

If you hide from the GAL then nobody will be able to email it.

no need to hide it man
you need only to configure some user to send to this DL, you can do that through the bellow steps:

http://exchangeserverpro.com/restrict-distribution-group-exchange-server-2010

its mentioned for exchange 2010 but its the same in exchange 2007 also

for more information about the topic check this on microsoft technet

How to Configure Message Delivery Restrictions

http://technet.microsoft.com/en-us/library/bb397214(EXCHG.80).aspx

Which is exactly what I have already said.

Transport Rules?

http://technet.microsoft.com/en-us/library/bb124703.aspx

You cannot use transport rules in Exchange 2003!

Good job it's 2007 were discussing then.

Oops :) working in a similar question which is 2003 :)

Detail was minimal. No instructions on specifics to perform the task just a general answer.

What do you mean no specifics given? I even gave you where to find the setting.  Quote from original post "You can under mailflow settings setup the group to reject mail from this you haven't given permissions to."