Input Credit avail

Dear Sir/Madam,

one of my client who has purchased goods from many registered dealer say around 5-6 crore during the financial year but he often make payment on account to some of suppliers.

My query is that whatever in GST portal has auto generated system to block ITC credit avail if 180 days is over or
only registered dealer who can blocked ITC credit.

Please clarify my doubts and also provide some references material for understanding me clearly.

Thanks in Advance