If this happens on eclipse start, it is maybe not the dependecy update of your project but the Maven repository index update (as khmarbaise mentioned in his comment).
You can disable this here: Preferences → Maven → Download repository index updates on startup
A fresh index offers you an up to date list of dependencies, e.g. in the Add Dependency dialog. But I found it will do if the index is updated manually (as needed) in the Maven Repositories View.
Update: Since Eclipse Luna the index update is now disabled by default (see Bug404417).