SmallDog won't leave BigDog alone.

[GallumDrawnAndQuartered]

and it's going to end in tears.

He keeps hassling him constantly. licking him, sniffing him, nudging him etc.

BigDog doesn't want to play and is getting stressed by the constant attention.
What do I do?

Seperate them. Either different rooms, one in a crate or small dog on a long lead where you physically reel him away from bigdog. Isn't fair on bigdog otherwise

Have shut smalldog in the kitchen for a bit. poor smalldog - he loves and adores bigdog but bigdog just doesn't want to play.