SB 4.7.17

vaiṣṇavaṁ yajña-santatyai
 tri-kapālaṁ dvijottamāḥ
puroḍāśaṁ niravapan
 vīra-saṁsarga-śuddhaye
Synonyms: 
vaiṣṇavam — meant for Lord Viṣṇu or His devotees; yajña — sacrifice; santatyai — for performances; tri-kapālam — three kinds of offerings; dvija-uttamāḥ — the best of the brāhmaṇas; puroḍāśam — the oblation called puroḍāśa; niravapan — offered; vīra — Vīrabhadra and other followers of Lord Śiva; saṁsarga — contamination (doṣa) due to his touching; śuddhaye — for purification.
Translation: 
Thereafter, in order to resume the activities of sacrifice, the brāhmaṇas first arranged to purify the sacrificial arena of the contamination caused by the touch of Vīrabhadra and the other ghostly followers of Lord Śiva. Then they arranged to offer into the fire the oblations known as puroḍāśa.
Purport: 

Lord Śiva’s followers and devotees, headed by Vīrabhadra, are known as vīras, and they are ghostly demons. Not only did they pollute the entire sacrificial arena by their very presence, but they disturbed the whole situation by passing stool and urine. Therefore, the infection they had created was to be first purified by the method of offering puroḍāśa oblations. A viṣṇu-yajña, or an offering to Lord Viṣṇu, cannot be performed uncleanly. To offer anything in an unclean state is called a sevāparādha. The worship of the Viṣṇu Deity in the temple is also viṣṇu-yajña. In all Viṣṇu temples, therefore, the priest who takes care of the arcanā-vidhi must be very clean. Everything should be always kept neat and clean, and the foodstuffs should be prepared in a neat and clean manner. All these regulative principles are described in The Nectar of Devotion. There are thirty-two kinds of offenses in discharging arcanā service. It is required, therefore, that one be extremely careful not to be unclean. Generally, whenever any ritualistic ceremony is begun, the holy name of Lord Viṣṇu is first chanted in order to purify the situation. Whether one is in a pure or impure condition, internally or externally, if one chants or even remembers the holy name of the Supreme Personality of Godhead Viṣṇu, one immediately becomes purified. The yajña arena was desecrated by the presence of Lord Śiva’s followers, headed by Vīrabhadra, and therefore the entire arena had to be sanctified. Although Lord Śiva was present and he is all-auspicious, it was still necessary to sanctify the place because his followers had broken into the arena and committed so many obnoxious acts. That sanctification was possible only by chanting the holy name of Viṣṇu, Trikapāla, which can sanctify the three worlds. In other words, it is admitted herein that the followers of Lord Śiva are generally unclean. They are not even very hygienic; they do not take baths regularly, they wear long hair, and they smoke gāñjā. Persons of such irregular habits are counted amongst the ghosts. Since they were present in the sacrificial arena, the atmosphere became polluted, and it had to be sanctified by trikapāla oblations, which indicated the invocation of Viṣṇu’s favor.