As far as I know, there is no mean to prevent an addon to be installed if it conflicts with another.

I can't find the discussion but if I remember, OpenERP SA considers that all the modules should be compatible between them.

I think that's not realistic, we'll always have edge cases where an addon will be conflicting with another.

The least we can do is to indicate the conflict in the addon's manifest (__openerp__.py) for informational purposes. The better would have OpenERP to be able to handle a conflicts declaration in the manifest as you said.