If you are planning to migrate a site collection in SharePoint to a newer version or simply lock site collection for some maintenance, you can do it through Central Administration. Remember this is possible only at site collection level.
Go to Central Admin -> Select Application Management -> Under Site Collections select Configure quotas and locks
- Adding content prevented – prevents users from adding new content, old content can still be accessed and edited
- Read-only(blocks additions, updates and deletions)
- No access – prevents access for every user. Even for site collection admin. Use this if you want to lock site collection.
You also have quotas setting available here to set max storage email alert settings.
This setting can be accessed at same location in Central admin for SharePoint 2010 and 2013. In MOSS 2007 it is available at
Central Admin -> Application Management – Under SharePoint Site Management select Site collection quotas and locks
Article from: http://sharepointviews.com/lock-site-collection-or-make-it-read-only-in-sharepoint/