The word ''Pasandi'' comes numerous times in the Chaitanya Charitamrita while ISKCON uses the word to describe ''devotees of demigods''. What is the meaning of this word according to the scriptures?
Pashanda (पाषण्ड) according to Sanskrit Dictionary means one who is heretical or impious or a person who does not follow the Vedas or a person who falsely impersonates a Brahmanical/Vedic persona.
It's also a word denoting non-brahmanical ascetics like Buddhists or Jainas. Anybody that follows a false doctrine or is heretical in his views.
Since you're asking for scriptural references I'm quoting Manu Smriti Section 4.30
पाषण्डिनो विकर्मस्थान् बैडालव्रतिकान् शठान् । हैतुकान् बकवृत्तींश्च वाङ्मात्रेणापि नार्चयेत् ॥ ३० ॥
pāṣaṇḍino vikarmasthān baiḍālavratikān śaṭhān | haitukān bakavṛttīṃśca vāṅmātreṇāpi nārcayet || 30 ||
He shall not honour, even with speech, impostors, those who follow improper occupations, those who are cat-like in their behaviour, hypocrites, logicians, and those who behave like herons.—(30)
Here the verse is translated as an "imposter". The explanatory notes say this:
‘Pāṣaṇḍinaḥ’—‘Ascetics who wander about with external marks, such as nakedness, red-dresses, and so forth’ (Medhātithi, who does not explain the term as ‘non-brahmanical ascetics,’ as asserted by Buhler,—and also Govindarāja);—‘Śākyas, Bhikṣus, Kṣapaṇakas and other ascetics outside the Vedic pale’ (Kullūka and Nārāyaṇa);—‘those who do not believe in the Vedas’ ‘pāṣaṇḍinaḥ’ as ‘those-who have recourse to such life-conditions as are opposed to the dictates of the Vedas—