Yes, as a matter of freedom of speech. Opponents of an initiative or referendum can certainly express the opinion that it would not be a good idea for a voter to sign a petition. An opponent, however, does not have the right to interfere with the petition process. In fact, it is a gross misdemeanor to interfere with somebody else’s right to sign a petition, and there are also laws against assaulting people. You can certainly express your opinion, but you must remember that other people have rights to their opinions as well, including the right to sign petitions you may not like.