[Simon Lloyd]

You'd have to do it in phpmyadmin (database) then you'd need to edit the modifyoptions template and find this

<tr>
							<td><br />$vbphrase[can_email_when_sends_you_pm]</td>
						</tr>
						<tr>
							<td><label for="cb_emailonpm"><input type="checkbox" name="options[emailonpm]" value="1" id="cb_emailonpm" $checked[emailonpm] />$vbphrase[receive_email_notification_pm]</label><input type="hidden" name="set_options[emailonpm]" value="1" /></td>
						</tr>

and remove it

EDIT: the code above is for vb3 but just look in the same template for similar code and remove it