If you want to tighten the spy sunglasses, you could just use the screwdriver to make the screw in the frame side and the nose pad tight. If you don't know how to do it, you could go to the eyeglasses store to let the client help you do this. It will be so easy.
Well, it seems to me that the spy sunglasses you have bought does not suit you very well. So, there are simply no effective ways to achieve this goal, however, if you try to take the sunglasses to some craftsmen, they will fix the sunglasses to better suit you. On the other hand, you can ask the seller for replacement, however, this would require some extra fees, but is is worth it.
Okay you can use a string, but that will look like a grandma, that's a joke, actually.
Here is my suggestion. First, go to the store which the sunglasses came from, then ask them to modify the frame for your, to make it suitable for you. Secondly, if you want to do it at home, you can use an appropriate spinner to tighten it.