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.