A window that you never intend to open can be permanently fastened shut. This could be done with plastic zip-ties, a window frame lock, or a screw or bolt through the window frame. A window that has been permanently disabled does not need window nets.
Some types of child lock can also be used, like this one:
Child-proof lock, firmly attached to window frame and handle, limiting the opening distance to less than 3cm
There are two requirements:
The lock has to be firmly attached, so that it cannot fall off, or be loosened over time
It has to be able to secure the window less than three centimetres open. Some child locks leave a gap which is far too small for children, but easily big enough for a cat.
Again, we will require photographs of the child lock. Not all windows are suitable for such locks.