When Yasser joined the company he currently works for, his employer made him sign an agreement that said that he could not join a union during his time of employment there. In other words, they made him sign a yellow dog contract, and in response,

Yasser's employers can be prosecuted.

Explanation:

The Norris-La Guardia Act (1932) outlawed yellow dog contracts. Yellow dog contracts try to force employees into agreeing to not join a labor union. Since yellow dog contracts are illegal, they cannot be enforced. If the employers try to enforce them, they can be prosecuted.