Dense casual-register sentence. 相手にされる ('be taken as a partner / be dealt with') is the passive of 相手にする, and その negated form ~相手にされなくなる means 'come to not be dealt with (by people).' ぞ is a strong masculine assertion particle — 'I'm warning you.' みんなから marks 'by everyone' as the agent of the passive. The ~なくなる construction captures the gradual onset of social exclusion as a consequence.