In short - yes, but it must be careful. The company has the right to express its opinions. However, the NLRA has rules that somewhat limit that freedom - companies cannot violate their employees' right to unionize if they so choose. It is often a fine line between expressing a policy preference for no unions, which is permitted, and telling employees they cannot unionize, which is illegal. If the company goes too far to discourage its employees from joining a union, it will violate the NLRA and may have to account for its actions to the NLRB.
Companies that do not want unions in their workplace often go to great lengths to discourage union activity, hiring unionbusters, professional consultants or lawyers who specialize in advising employers on how to thwart union organizing drives or how to decertify unions. Unionbusters usually self-identify as union avoidance firms, management consultants, or labor consultants. Unionbusters help an employer create a sense of dissension and division among employees during an organizing campaign and spread misinformation about the union before workers vote in a union representation election. For more information, see the Jobs with Justice page Unionbusters 101.