How to configure my rear suspension I recently bought a second hand bike, that got through quite some repairs and testing at a nearby bike store. They fixed some issues and put the bike into cycling condition, as well as changing the tires.

After some travelling, I have noticed that on very bumpy roads the rear suspension starts making what i would label as "B-series movie bed suspension noise" (niieek, nieek, nieek), the spring seems to be "jumping" too much and provoking the scratchy sounds.

I have also noticed that the rear suspension has some kind of screw mechanism to strenghten or loosen the spring (see pic), and that it's halfway, but I have no idea how to touch it and I don't want to trouble the bike store again for such a simple thing. 

I understand that what I should be looking for is to increase the strength / stiffness of the suspension, to reduce the scratching, at the (relaxing for my ears, worse for my own rear) cost of less shock absorption.
The squeaking is caused metal parts rubbing against one another, not by the spring 'jumping'. Maybe you mean the spring is rotating on the shock body as it is compressed and uncompressed?

Adjusting the suspension is not the answer to getting rid of the noise, if you did the noise would probably still be there.

BTW, the adjustment you are talking about is the preload of the suspension spring. Here is a question about that (for forks, but the principle is the same).