Doing this would be a "core" change, which is allowed, but not always recommended.

In the desktopmodules/admin/security folder there is a file called securityroles.ascx

You want to modify

<asp:CheckBox ID="chkNotify" resourcekey="SendNotification" runat="server" CssClass="SubHead"
                Text="Send Notification?" TextAlign="Right" Checked="True"></asp:CheckBox>

setting Checked="False"

the thing to remember with a core change is, when you upgrade DNN that change will likely get lost, and you'll have to make it again.

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Chris Hammond
Former DNN Corp Employee, MVP, Core Team Member, Trustee
Christoc.com Software Solutions DotNetNuke Module Development, Upgrades and consulting.
dnnCHAT.com a chat room for DotNetNuke discussions