On P99, slamming as a Shaman does not stop a fleeing mob. I recall on live slamming a fleeing mob often stopped them briefly.
http://web.archive.org/web/200102010.../eqshaman.com/
Slam has the ability to disrupt an enemy caster. If you are in melee, slam will only do 1 point of damage if it hits, but even that is enough to interrupt an enemy caster. At level 20, the slam skill also gains the ability to stun, which can be very helpful dealing with fleeing mobs without wasting mana or in causing a short pause in the mob’s attacks