How to configure odata Service to be readonly for technical user

[nerevar]

Context : 

I have a simple odata service in C4C to access the customers data.

I created a system user and a communication arrangement to be able to access it with certificate authentication.

I would like to be able to query the odata with get requests. But I don't want to enable other actions.

Is it possible? how to do it?

From what I know if  restricting "write" possibilities is only available in workcenter view of functionnal users.

Thanks for your help

[nerevar]

So actually I found the option in the Communication arrangement parameter but I could not see very well because the field is a bit hidden in the screen