Infrequent Buyers

Just wondering if anyone else has a problem with buyers who haven't bought anything for the last 6-12 mths then purchase and never contact or make payment .I seem to be getting quite a few lately.I have blocks in place but these bidders all have good feedback and some are sellers,can you block someone who hasn;t bought for the last year.I realise not all are tarred with the one brush but I seem to get my share.