I doubt there's much you can do about this. This seems to me like a problem with the oven itself, more in particular the type of material that was used to cover the insides with.

Maybe you can try greasing the bottom of your oven with something like olive oil or something? No idea if this will help though...

Alternatively, you can try a roasting pan. This will prevent the grease from the food from dripping onto the bottom of your oven, effectively solving your problem. As you can see, it's not an expensive solution either. I'm using one of those things and I like it very much. It's easy to clean and very handy to use.