Spam Bounce:

A spam bounce indicates that at the time you emailed the subscriber, the message was blocked by a spam filter. The filter could be either on the subscriber's end or ISP due to a domain block, large images in your creative, inappropriate content, too many special characters or ALL CAPS in the subject line or because of a listing on an anti-spam site, like Spam Cop.

Because a spam bounce is normally temporary and it is not a hard bounce, they are removed from your active subscriber list and quarantined in the Manage Bounces section, but only for 48 hours. After 48 hours, the spam bounce is added back into your subscribers list. You are able to batch or individually delete these addresses.