You can ask him to leave at any time, or simply not recruit him? You can also callh im otu many times in the dialogues, so how can you not stop it?
It seems you mistake preventing something and stoping it ...
You see if you not recruit him, or dont talk to him ... there is nothing to stop, since his lies were never there in the fisrt place.
In addition, this is hardly even valid reaction ...
Ignoring holes in Wylls story they dont dissapear, he dont sudently become perfectly written character.

He's a bit of a narcassist and it works great.
His narcisim is not a problem ... the problem is "
well kept secret" as with all others ...
Only blind man would not see that Astarion have suspiciously pale skin and pointy teeth ...
Shadowheart have her whole body covered by Obsidian and suspiciously many snarky comentaries and Selune, the main rival or her Goddess ...
And Wyll? Through your connection you know about Myzora since your first meeting, you can feel magic in his eye, he keep talking to himself as when he is talking with someone else (wich we know he does) ...
The problem isnt they does those things ... okey, if Larian wants to create idiots who will tatoo on their forehead "i am serving evil God" and then pretend they are good characters, fine its their bad decision ...
But forcing our characters to accept such obvious and stupid lie IS bad design.
That is what Niara (unless i missunderstand her) is complaining about and i completely agree with her.
The funny part is how easily it could be resolved:
Tav: "I dont believe you."
Character: "Well too bad for you, since im done talking about it."
*End conversation.*And voila!
Your character is no longer forced to be thrusty idiot who accepts anything people tells him.
Simple isnt it?

//Edit: Yeah ... basicaly what GM said ...
//2. Edit:
One more think i forgot to say ...
Personaly i dont think that their characters are necesarily "bad" ... its more like our lack of options to smash it over their own face that pisses me off.
Actualy i believe such attitude would work well for Wyll ... eactly bcs he is such smug, narcisist, and self-interested moron ... that he WOULD actualy believe that nobody can ever know his dark secret, bcs he is "so good" in hiding it.
That would actualy fit the character perfectly.
The problem is that our character HAVE TO play along ... and that is what i dislike.
So if Larian would simply add us here and there some reaction that would be something like:
"
What kind of moron you think i am to believe such bullshit?"
ESPECIALY after our character SUCEED in Arcana check, so s/he LITTERALY KNOWS there is something magical in his eye ... and it is most definietly not "Just a pebble." as he claims, convinced that nobody could ever see through his "masterfull lie".
Then the whole situation would be much better.
