You could nuke your mods ability to delete shouts by editing shoutdelete.inc.php (scripts directory, and completely untested):

Change
Code
	if ($user_id == $user['USER_ID'] || $user['USER_MEMBERSHIP_LEVEL'] == "Administrator" || $user['USER_MEMBERSHIP_LEVEL'] == "Moderator") {

To
Code
	if ($user_id == $user['USER_ID'] || $user['USER_MEMBERSHIP_LEVEL'] == "Administrator") {

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