Just like this time, Liu Yu could have simply pretended that Gongsun Zan never appeared; after all, the man was consuming his own provisions. Logically speaking, Liu Yu had no right to obstruct Gongsun Zan, yet Liu Yu was the Governor of Youzhou, wielding the utmost power within the Youzhou region!
Now Gongsun Zan was in a pitiful state. Liu Yu sized up his own frail arms and legs, then considered Gongsun Zan, who had been battered by Yuan Shao, and was sure that if he kicked Gongsun Zan while he was down, Gongsun wouldn't be able to do anything about it. Therefore, he planned to properly educate Gongsun Zan on how to behave.
This goes to show that in reality, Liu Yu was also quite sly, except Gongsun Zan had simply never given Liu Yu the opportunity to show it before.