While there's no canonical way to contact a user, github recently introduced a Mention @anybody, they're notified feature.
When you @mention a GitHub username anywhere in the context of an issue or pull request, that person is notified and subscribed to future updates.
You can also create a gist & mention him in the comment, thereby notifying the user.
You could define different groups of labels like issue types, issue priorities, issue statuses, version tags, and maybe more. In order to be able to see instantly to which group a label belongs to you could use a naming convention like :.
Using such a naming convention should make managing Github issues much easier and helps others to "understand" issues much faster. Note that you can also assign colors to labels which can add even more to readability (I would use a specific color for each label group). But because you still have to assign/unassign those labels to/from issues manually you might want to keep the overall list of groups/labels small.
According to the scheme suggested above you might define groups and corresponding labels as follows.
'issue type' group
type:bug
type:feature
type:idea
type:invalid
type:support
type:task
'issue priority' group
prio:low
prio:normal
prio:high
'issue status' group
(These labels describe an issue's state in a defined workflow.)
status:confirmed
status:deferred
status:fix-committed
status:in-progress
status:incomplete
status:rejected
status:resolved
'issue information' group
info:feedback-needed
info:help-needed
info:progress-25
info:progress-50
info:progress-75
'version tag' group
ver:1.x
ver:1.1
Best Answer
I did my own experiment, here is what I found:
If you block a user, they will be prevented from commenting or opening issues on any repos that you own
For any repos where you are a collaborator, they are unencumbered
Also I found this:
http://help.github.com/articles/blocking-a-user-from-your-personal-account