Set the pre-existing content  databases to off-line. Users will still be able
to access the site and upload documentation but new sites will not be able to be
created. Also, if you drill into the content database itself, there's an
option to set the maximum number of sites that it can contain.

This sounds exactly like what I am trying to accomplish. Do you know of any
additional resources that I can learn more about this…I would like to set this
up for several of our content  databases.

I advise against setting the databases  offline, there are some unintended
consequences by doing so, such as disable the profile synch. Setting the max
sites a content  database can contain is the right way to do.

Also SharePoint 2010 has lots of improvements with regard to this, see
www.toddklindt.com/.../Post.aspx?ID=205

For clarification… setting a database to offline will prevent new Site
Collections from being added to the database, not sites. If users have
permission to create subsites, they will still be able to do this in an
off-line database. Max sites a database can hold also applies to Site
Collections, not sites.

There's no great way to do this. Setting the database offline has the drawbacks
you mentioned, it breaks profile sync, among other things. Setting the Max value
is a good way, but it takes a lot of work. That method breaks as soon as you
delete any site collections. So every time you create a site collection you have
to make sure every database's max and current are the same. Kind of a pain.