You can also block the instance at the account settings level (Blocks tab). That way you will not see posts from these instances on both the webUI and mobile apps.
Sadly it doesn't have a toggle to block comments and post from users of these instances made on communities outside of these blocked instances
Mine doesn't have @ signs. This might be easier to do in the lemmy web UI than within Sync. When you start typing an instance name, it will pop up a list of matches that you can click, so you don't have to worry about typos or syntax.