After several days without internet I could do a little more research and solve my problem. (connectivity in my country is a nightmare)
Thanks the tip about "pin a package" and thanks Google and this forum I discovered that I could create a file called /etc/preferences where I can set a priority to sources and/or packages
I have created the file /etc/apt/preferences with the following text
That tells apt that the Medibuntu repository have lower priority than others repositories. Then, if a package exist in other repository apt will try to install (or update) from them... if I request a package that only exist in Medibuntu, then will be installed from Medibuntu.
Pin: release l=Medibuntu
More information can be found goggling for "apt pin"
A site that was very useful for me is: http://www.debian.org/doc/manuals/ap...pt-get.es.html
Finaly, thank bapoumba for your assistance and patience.