OG : disable automatic email when a new member is made admin of group

I'm using organic groups on my Drupal installation and I would like to allow some group members to edit other people's contributions to the group, while others could only view them.

I do not want people to edit node that are not part of a group they are in, unless its their own contribution.

So what I was thinking :
*Set users roles so they can only edit their own contribution
*Have nodes set to "Standard group post (typically only author may edit)"
*Make the desired members group administrator, so that they can edit everyone's contributions to that group, while leaving some members to read-only for other people's contributions

All well and everything - except : When i set a member as group administrator, they automatically receive an email saying so. I do not want all the members to get that email, since its not relevant that they know it, and I don't want to flood their inbox. I already have over 50 members.

I tried editing a user's profil to "No email notification", making sure all his notifications were at "never", but the user always gets the email.

I do not have access to the server, it is a hosted solution, so playing with files isnt really possible. Installing a new module might be possible but would have to be justified.

My low-tech option is to just change the email text to something like : "maintenance on the site, please ignore this email".

Anyone have other ideas?