I believe this fogging used to be a common problem of scopes in earlier days. It is a mechanical problem to be solved -- perhaps involving sealing a neutral gas, such as nitrogen, inside the scope under positive pressure relative to the outside atmosphere and sealing "permanently" -- which most manufacturers HAVE solved, but perhaps only for the more expensive scopes.

If your manufacturer cannot repair this at a satisfactory price or under warranty, your best bet is just to buy a better scope next time. Then again, maybe your scope is a high quality scope and it is just an exceptional case of failure?