Most helpful. Thanks to you both. My confusion came from the fact that the so-called active verb 속다 has a "passive" interpretation in English, i.e. 속다 = get/be fooled (deceived).
Its passive-causative form (as Elise has pointed out) is 속이다, which means to get someone (be) fooled; hence, to fool someone, consequently adopts an active voice.
In this example, selma has correctly explained that the 속이 from 속이지 마다 is the infinitive of 속이다.
In short, you are both right. Thanks once more.