Mercutio makes one pun from the word "prick" in his lines,
If love be rough with you, be rough with love.
Prick love for pricking, and you beat love down. (I.iv.28-29)
The term "prick" can be used to mean "to pierce," or to "puncture," especially to cause pain (Random House Dictionary). However, since it can also refer to an "erected" object, like a dog's ear, Mercutio is also using the term with sexual connotations (Random House Dictionary).
Mercutio forms two other puns from the word "done." When Romeo continues to refuse to join his friends in crashing the Capulet's ball, saying, "The game was ne'er so fair, and I am done," meaning I am finished playing these silly games (40), Mercutio responds, making his first pun out of "done," with, "Tut, dun's the mouse, the constable's own word!" (41). While Romeo, in saying that he is "done," actually means finished, Mercutio makes a pun by rhyming the word "dun" to refer to the adjective "dun," which can describe a "gray-brown color," just like the color of a mouse. Historically, the expression, "dun's the mouse" meant "be as quiet as a mouse" ("Romeo and Juliet," shakespeare-navigators). Beyond that, Mercutio's phrase, "the constable's own word," also refers to the historically understood concept that the constable of a town "sat around" quietly "doing nothing." Hence, through the pun of "dun" referring to the silence of a mouse, just like the constable, Mercutio is chastising Romeo for sitting around and doing nothing ("Romeo and Juliet," shakespeare-navigators).
The second pun Mercutio makes by rhyming the word "done" with "dun" is in his line, "If thou art Dun, we'll draw thee from the mire" (42). In this line "Dun" refers to a historic game called "Dun the horse." The game was played at Christmas time and consisted of pulling a log out of mud ("Romeo and Juliet," shakespeare-navigators). Hence, Mercutio is saying that if Romeo is truly "done" then, due to his lovestruck behavior, he has also allowed himself to sink into "mire," or a poor state of mind. Mercutio is claiming that if Romeo is truly "done" then they will pull him out of the "dun," or mud, or mire.
Mercutio has to be one of the Punniest of all of Shakespeare's plays. In fact, the purpose of Act I, sc. iv, is almost exclusively focused on Mercutio's ability to be the sorely needed comic relief in The Tragedy of Romeo and Juliet.
Romeo is either depressed or over-the-moon in love; his emotions are as changeable as the weather. While his cousin Benvolio loves Romeo and attempts to lighten Romeo's mood, Mercutio is the "cut-up", the friend who can always be counted on to make the simpleset of things, like walking to a masquerade, a comedic event.
Line 28, Mercutio says: Prick love for pricking and you beat love down.
Of course this is a pun as puns twist and spin the meaning of one word, or a group of words into another meaning--not always caught by the reader.
Prick can be defined as a verb, as in to puncture, but it can also be used as a locker-room synonym for the male genitalia. In that content, beating love down takes on a violent meaning.
The entire Queen Mab speech is a monologue that is an extended pun
Romeo (to Mercutio): I dream'd a dream to-night.
Mercutio: And so did I.
Romeo: Well what was yours?
Mercutio: That dreamers often lie.
Lie is used as a pun to mean telling mistruth, or to lie down, as one does when sleeping.
I hope this helps.