Currently Nexus automatically blocks a repository when the repository sends back an HTTP 409:
2016-04-17 14:03:47,066+0000 WARN [qtp606893651-77164] anonymous org.sonatype.nexus.proxy.maven.maven2.M2Repository - Remote peer of proxy repository "Grails Plugins" [id=grails-plugins] threw a org.sonatype.nexus.proxy.RemoteStorageException exception. Connection/transport problems occured while connecting to remote peer of the repository. Auto-blocking this repository to prevent further connection-leaks and known-to-fail outbound connections until administrator fixes the problems, or Nexus detects remote repository as healthy. - Cause(s): The method execution returned result code 409 (expected 200). [repositoryId="grails-plugins", requestPath="/com/sap/ids/com.sap.ids.mongodb/2425.0.0/com.sap.ids.mongodb-2425.0.0.pom", remoteUrl="https://repo.grails.org/grails/plugins/com/sap/ids/com.sap.ids.mongodb/2425.0.0/com.sap.ids.mongodb-2425.0.0.pom"]
It seems that artifactory uses the code 409 while obtaining some artifacts. Given the nature of 409 errors, it would make sense to not block a repository when a 409 is received (as it's specific to one resource in particular.
The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body SHOULD include enough
information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user agent to fix the problem; however, that might not be possible and is not required.