I was under the impression that tracing glyph ward on a player who is aiming a ranged weapon or targeting a spell at you was supposed to make their attention waver from you. It works fine for ranged weapons but doesn't work at all against spells being targeted. The messaging looks like it worked, but it doesn't actually prevent casting the current spell or targeting/casting any subsequent ones. Tested using a big paladin against a highly outclassed ranger and others. Paladin had pristine soul state.

tar sta 10 at <Paladin>
That won't affect your current attunement very much.
With calm movements you prepare your body for the Stampede spell.
You begin to weave mana lines into a target pattern around <Paladin>.
<Paladin> traces a glyph in the air before you, and your attention wavers from him.
Your formation of a targeting pattern around <Paladin> has completed.
cast
You raise your right hand skyward and make a quick whipping motion.
A thunderous noise builds in the distance, increasing in cadence and intensity as it hammers into the area!
A ring of orange-hued mist bursts from the surface of <Paladin>'s pavise shield.
A wisp of orange-hued mist bursts from the edge of <Paladin>'s pavise shield.
A ring of orange-hued mist bursts from <Paladin>'s left arm, leaving him mostly unfazed.
Roundtime: 1 sec.
Dark golden light glares forth from <Paladin>, lightly stunning you as a sonorous voice proclaims, "Retribution!"
tar sta 10 at <Paladin>
That won't affect your current attunement very much.
With calm movements you prepare your body for the Stampede spell.
You begin to weave mana lines into a target pattern around <Paladin>.
You feel fully rested.
You feel fully attuned to the mana streams again.
Your formation of a targeting pattern around <Paladin> has completed.
cast
You raise your right hand skyward and make a quick whipping motion.
A thunderous noise builds in the distance, increasing in cadence and intensity as it hammers into the area!
Orange-hued rings of mist burst from the ground near <Paladin>.
Orange-hued rings of mist burst from the ground near <Paladin>.
Roundtime: 1 sec.

The bug seems to have existed since 2012, altough the messaging is now different:
http://www.tinyheroes.com/forums/DragonRealms/DragonRealms%203.0/Paladins/thread/1515359

That would make sense, but after glyph ward was traced during targeting it said the targeting pattern was completed. Also, it didn't prevent targetting the paladin again immediately afterward, and that targetting pattern also completed. For ranged weapons, the first aim fails and you never get to "You think you have your best shot possible now." Also, aiming is prevented over a period of time, and if you try to aim it says you're unable to focus on the paladin. This doesnt seem to be the case with targeted spells, unless it's just a messaging problem; However, the paladin had twice the ranks in shield as the ranger's TM, and hundreds more ranks in evasion, so the fact that the first stampede was able to hit makes me believe it was behaving as fully targeted and not snap cast. Sorry if I'm wrong here and I'll do some more testing but so far it seems to me like there's no effect on targeting spells.

Ah, thanks for clearing that up, the messaging was confusing me. Haven't tried it in actual PvP, only in testing, but thanks for the tip. Have been searching for information about it but could only find the post I linked. Thanks!