From: Secret of #SharePoint – Tip of the Day

If a site collection is growing to large or you for other reasons want to prevent users from adding new content, you can set a site collection lock on the site so that only existing content can be modified.

This can be done either using PowerShell or from Central Administration. In SharePoint 2010 you find this setting in Central Administration by going to Application Management and Site Collection, quotas and locks.

This tip provided by Expert in Residence, Mattias Karlsson