Simple, you tell them something like Zobia does to me…“shuker kero mera mood ajeebsa hay nahi to abhi batati tumhain!! 
”
And those whom I dislike I do not avoid them, infact i do try to befriends with them. Agger kissi ko avoid kerna hi hay to bhaye jaan aap nay dosti hi kiu ki thi…Ya phir aap uss banday ke saath hi kiu ho…
A fake smile is hard to practice, and a fake humour. For example: If someone pulls up a joke upon me when I am not in the mood. (heh..as if) I usually look at them and smile. And some how they understand that they did that at the wrong place at the wrong time.
Or I’d give them a phrase “very funny!! :D”