Both affirmative and negative formal (usted & ustedes) commands use the present subjunctive verb form. In the affirmative form, the reflexive pronoun is joined to the imperative form but in the negative form, the reflexive pronoun goes before the verb.
Spanish: No se bañen.
English: Do not bathe yourselves.
To conjugate "bañarse" in the negative "ustedes" command form, you would use the subjunctive present form. The negative command "no se bañen" would be the conjugation for "bañarse" in the "ustedes" form.