Outdoor /Indoor

[H&SWells]

I am getting new kittens in April who will be indoor cats - with an outside cat run - but my moggie had full access to the outside via a cat flap - I am sticking on the point of how to stop the kitties from using the cat flap as my moggie is 16 i dont really want to have to trat locking it on her as she is going to have enough change to deal with.....any suggestions or has anyone else been through this.