Make your SharePoint 2013 site collection read-only, restrict delete

You can apply locks to a site collection to prevent users from updating content or you can temporarily prevent users from accessing the site collection.

1. Open SharePoint central administrator and click on Application Management

2. Click on Configure Quota and locks, new wizard will be open here

3. Select Lock option like below and click on ok

4. Then Open SQL studio and connect DB Engine

5. Find out DB name of site, CA>Application management> View Site collection and select on site

6. Set SharePoint Content Database in Read-Only Mode, SQL Server Management Studio. Right-click on the database -> Options -> Make database read only -> True

Now site will be showing as Read only