No, you cannot void the contract unless by "pressure" you mean threats of force, blackmail, or other illegal actions. The law does not make being "nagged" or made to feel uncomfortable or being repeatedly asked to do something (e.g. sign a union contract) grounds to void a contract, since you could still have freely chosen to ignore the requests and told everyone leave you the [fill in your favorite curse word] alone. Since you signed it when you had the power to not sign, the law holds you to what you signed.