Update: I should have probably started with this as your projects are SNAPSHOTs. It is part of the SNAPSHOT semantics that Maven will check for updates on each build. Being a SNAPSHOT means that it is volatile and subject to change so updates should be checked for. However it's worth pointing out that the Maven super POM configures central to have snapshots disabled, so Maven shouldn't ever check for updates for SNAPSHOTs on central unless you've overridden that in your own pom/settings.
You can configure Maven to use a mirror for the central repository, this will redirect all requests that would normally go to central to your internal repository.
In your settings.xml you would add something like this to set your internal repository as as mirror for central:
<mirrors>
<mirror>
<id>ibiblio.org</id>
<name>ibiblio Mirror of http://repo1.maven.org/maven2/</name>
<url>http://path/to/my/repository</url>
<mirrorOf>central</mirrorOf>
</mirror>
</mirrors>
If you are using a repository manager like Nexus for your internal repository. You can set up a proxy repository for proxy central, so any requests that would normally go to Central are instead sent to your proxy repository (or a repository group containing the proxy), and subsequent requests are cached in the internal repository manager. You can even set the proxy cache timeout to -1, so it will never request for contents from central that are already on the proxy repository.
A more basic solution if you are only working with local repositories is to set the updatePolicy for the central repository to "never", this means Maven will only ever check for artifacts that aren't already in the local repository. This can then be overridden at the command line when needed by using the -U switch to force Maven to check for updates.
You would configure the repository (in your pom or a profile in the settings.xml) as follows:
<repository>
<id>central</id>
<url>http://repo1.maven.org/maven2</url>
<updatePolicy>never</updatePolicy>
</repository>