In such case, what I do is to uncheck "Use CPU" in Primegrid Preferences.
This will make the server not send any CPU work, no matter what CPU subproject is checked.
I do likewise to stop receiving GPU work. (I have an NVidia card, so I uncheck "Use NVIDIA GPU".)
Next to "Use CPU" there is a note.
I understand that if you leave "Use CPU" checked and uncheck all CPU sub-projects, then "321-Sieve will be automatically selected for you.
Enforced by version 6.10+"
"Accidit in puncto, quod non contingit in anno."
Something that does not occur in a year may, perchance, happen in a moment.