How to restrict the downgrade to many applications from a new application

We had three applications called A, B and C. Now, combining the functionality of A, B and C a new Application X is developed.

We can add the major upgrade tables having the Upgrade code of A, B and C additionally in the installer of X so that if the user already has A, B and C installed in his machine, when he runs installation of X(X having the higher product version than that of A, B and C) the applications A, B and C will get uninstalled and X will get installed(upgrade).

but the problem lies in the case if the user tries to install A, B and C again after he had already installed X.we can't restrict the installation of A, B and C.

Can anyone suggest any workaround to restrict the installation of A, B and C(downgrade) after X is Installed.

Note that we can can note modify the installers for A, B and C now.

Thanks,
Vikram