MArtijn,

Following code sample shows how to retrieve a booking and assign a resource : Silently assigning resource, profile and parameter values | DataMiner Docs

If you use a development environment (ie Visual Studio), you can see that the ServiceReservation object used in the above-mentioned example also has a 'Unassign' method that can be used to remove a resource from a booking.

Then following code sample can then do the job :

var requests = new[] {
new AssignResourceRequest
{
NewResourceId = ResourceGuid
}
};

var sri = SrmManagers.ResourceManager.GetReservationInstance(ReservationGuid) as ServiceReservationInstance;

sri.UnassignResources(engine, requests);