Negative ASINs have no effect in Auto Campaigns
Although in theory it is possibly to set ASINs to negative, this will not have the desired effect in automatic campaigns. It will only stop your ad from appearing when customers actually enter the ASIN into the search bar.
Your ad will continue to be displayed on the detail page of the respective ASIN though.
Negative ASINs only useful for Category Targeting Campaigns
Where it does make sense to use negative ASINs, however, are (manual) product targeting campaigns - in particular for the category targeting option.
Here sellers will typically want to set very badly performing ASINs to negative, or ASINs that made sales and were thus moved to another manual product targeting campaign.