LoL, quite a few things here:
1. Moderators and Supermods can only "remove" warnings. Remove means "artificially mature" sooner than the normal maturity date.
2. Only Admins can "Completely Remove" a warning, i.e. make it look as if it was never issued.
What you want is that mods and supermods cannot even "artificially mature" a warning?
If so, you need to add those conditionals in the templates, and in mod_warn.php, find:
$y="<a href='{$vboptions[bburl]}/Warn.php?do=RemoveWarning&id={$warn[wid]}'>Remove</a>";
Replace that with:
$y='';
That will remove the ... "Remove" link from the modcp table.
To do it more properly, I need to come up with a more elaborate permissions scheme, where you define what the smods and mods can and cannot do. However, given that vBulletin 3.5 is out, and a lot of people have asked for this hack to be ported to 3.5, I have too much work to do, just checking if this can be done, so I do not have time for such changes in the existing version.
Rgds