¡No, no la pongan!, which is a (negative) command for "ustedes"; instead of "No, no la van a poner", which is a statement.
The latter can also be written with the direct object pronoun in a different position: "No, no van a ponerla"; but it'd still be a statement, and not a command, which is what the instruction requires of us.
If the instruction changed the question to "¿Vamos a poner la cebolla EN LA SOPA?", the uds. command, in the negative should be: ¡No, no se la pongan! (todavía, not yet, don't put the onion in it [the soup] yet)
In the command, there is NO NEED to say "ustedes" (No la pongan uds.), as it is grammatically unnecessary and further it can cause some confusion in Spanish: i.e. a Spanish-speaker might understand this: "Don't you guys add the onion to the soup, you guys, ustedes, whereas I (or they) can do it, but not you guys!".