Thank you Stéphane,
that did it.
Just to collect the information together (for me later and maybe others):
How to configure rspamd: https://rspamd.com/doc/quickstart.html#manual-configuration
Default (stock) configuration of modules is in /etc/rspamd/modules.d/module_name.conf
Custom settings in /etc/rspamd/local.d/module_name.conf
will be merged with stock configuration (default configuration)
Custom settings in /etc/rspamd/override.d/module_name.conf
will override the default configuration.
enabled = false
disables the module.
There is no extra nethserver configuration for rspamd, we just use the rspamd method as documented at https://rspamd.com/doc/
After changing configuration you have to restart rspamd: systemctl restart rspamd
Uwe