git config remote.name.annex-sync can be used to control whether a remote gets synced.

Command/Sync.hs

@@ -26,6 +26,7 @@ import Git.Types (BlobType(..))
 import qualified Types.Remote
 import qualified Remote.Git
 import Types.Key
+import Config
 
 import qualified Data.ByteString.Lazy as L
 import Data.Hash.MD5
@@ -71,7 +72,8 @@ syncRemotes rs = ifM (Annex.getState Annex.fast) ( nub <$> pickfast , wanted )
 					unwords (map Types.Remote.name s)
 			return l
 		available = filter (not . Remote.specialRemote)
-			<$> Remote.enabledRemoteList
+			<$> (filterM (repoSyncable . Types.Remote.repo)
+				=<< Remote.enabledRemoteList)
 		good = filterM $ Remote.Git.repoAvail . Types.Remote.repo
 		fastest = fromMaybe [] . headMaybe . Remote.byCost
 

Config.hs

@@ -86,6 +86,11 @@ repoNotIgnored :: Git.Repo -> Annex Bool
 repoNotIgnored r = not . fromMaybe False . Git.Config.isTrue
 	<$> getRemoteConfig r "ignore" ""
 
+{- Checks if a repo should be synced. -}
+repoSyncable :: Git.Repo -> Annex Bool
+repoSyncable r = fromMaybe True . Git.Config.isTrue
+	<$> getRemoteConfig r "sync" ""
+
 {- If a value is specified, it is used; otherwise the default is looked up
  - in git config. forcenumcopies overrides everything. -}
 getNumCopies :: Maybe Int -> Annex Int

debian/changelog

@@ -11,6 +11,8 @@ git-annex (3.20121010) UNRELEASED; urgency=low
   * webapp: Improve wording of intro display. Closes: #689848
   * webapp: Repositories can now be configured, to change their description,
     their group, or even to disable syncing to them.
+  * git config remote.name.annex-sync can be used to control whether
+    a remote gets synced.
 
  -- Joey Hess <joeyh@debian.org>  Wed, 10 Oct 2012 12:59:25 -0400
 

doc/git-annex.mdwn

@@ -795,6 +795,11 @@ Here are all the supported configuration settings.
   Or, it could be used if the network connection between two
   repositories is too slow to be used normally.
 
+* `remote.<name>.annex-sync`
+
+  If set to `false`, prevents git-annex sync (and the git-annex assistant)
+  from syncing with this remote.
+
 * `remote.<name>.annexUrl`
 
   Can be used to specify a different url than the regular `remote.<name>.url`