Well, you'd edit the editpost.tpl file; you should just be able to change this:
<input type="checkbox" value="1" id="markedit" name="markedit" checked="checked" class="form-checkbox" /><label for="markedit">{$lang.MARK_EDIT}</label>
<input type="text" name="editreason" class="form-input" />
To this:
<input type="hidden" value="1" id="markedit" name="markedit"/>
That SHOULD always set the value to "mark as edited". Please note that this is an unsupported (and untested) modification.