it seemed like Guinan, with her extreme age and listening abilities, had a 'special power' in that she knew the 'true nature' of the Q, and was not fooled by his tricks. Q might've just warned picard of her being an 'imp' in order to cast doubt on her in Picard's eyes, in an attempt to make her less credible (and in turn make himself more credible to picard)
in reality, it was probably a character development that the writers decided to drop and were luckily able to later attribute the whole thing to the Nexus and call it good
That...or maybe Q can only affect those who believe they are defenseless and that they CAN be affected. Perhaps Guinan was wise enough to realize this, and by showing she believed she could take a stand against him, he was thus unable to affect her. Which of course would anger Q to no end--especially if he couldn't do anything about it but cast aspersions on her credibility.