I have five PC's listed as participating in CPN. 3 of them I no longer have so I would like to delete them and have only my two active machines shown. I cannot find a way to do this.

You can't.
They'll disappear off the list of active computers after not being used for a month.

The reason they're kept, is because of the models they've run; If a researcher is looking through model data, he may need to know what sort of computer ran it.

That's fine. Thanks.