The following content is from Microsoft TechNet:

Some content databases are growing too large (SharePoint Foundation 2010)

Edit the rule definition to prevent new sites from being added to these databases, and then move some site collections to other databases.

1. Verify that the user account that is performing this procedure is a member of the Farm Administrators group.

2. On the Central Administration Home page, click Monitoring.

3. On the Monitoring page, in the Health Analyzer section, click Review rule definitions.

4. On the Health Analyzer Rule Definitions page, in the Availability category, click the name of the rule (Some content databases are growing too large).

5. In the Health Analyzer Rule Definitions dialog box, click Edit Item, and then select the Repair Automatically check box.

6. Click Save. You can no longer add new sites to databases that exceed 100 GB.

7. Move some site collections to smaller databases.  See

   How To Move A Site Collection To A Different Content Database for more information.