How can I stop SAS from using RCS while enabling it for the pilot?

While maneuvering I find it useful to have SAS point in a particular direction, e.g. pro-target. Using ijkl (or wasd in docking mode) one can then fine tune the prograde vector with thrusters. However, even if I have a strong electrical stability system, SAS still feels the need to use thrusters as well. This messes up all fine control as it jumbles around the prograde.

Some components such as torque wheels allow the wheel control to be switched from 'normal' to 'pilot only' to 'SAS only'. However, both monoprop thrusters as well as the Vernor thruster apparently only allow to toggle RCS in an all-or-nothing fashion.

Is it possible to tell SAS to only use reaction wheels even if RCS is enabled? Or is there another way to do what I want to accomplish? (without mods)

You can't set an RCS thruster block to "Pilot Only", but you can prevent it from handling rotation at all while still handling translation. Reaction wheels are usually far more convenient for handling rotation than RCS thrusters anyway, because they won't affect your velocity or burn precious monopropellant. So you often want the RCS thrusters to handle translation only and leave rotation control to the reaction wheels.

In order to do that, you first need to enable "Advanced Tweakables" in the Settings menu:

You can now determine what each RCS thruster will and will not do. Right-click on the thruster and click on "Show Actuation Toggles":

Then deactivate Yaw, Pitch and Roll:

You can do that while building a vehicle in the VAB/SPH or when the vehicle is in flight.