The Texture object implements Disposable so just invoke dispose before dereferencing it. http://libgdx.badlogicgames.com/nightlies/docs/api/
This is Java, so if you want to dispose of something you really just need to stop referencing it - BUT if that object implements Disposable, you should invoke this first so it:
A ) Doesn't have to wait for the GC to clean up it's background resources
B ) Releases unmanaged resources.
Just remember that after invoking dispose you can't use the object anymore.