The "set role" operator has a nice feature that lets you set the role of an attribute to free text.
By setting the role to something like "ignore" subsequent modelling operators will not process the attribute.
Here's an example that creates some fake data with 4 attributes and then sets three of them to be ignored. This causes the clustering operator to perform more poorly because it ignores some of the attributes.
Note that the roles have to be different so in this case they are "ignore01", "ignore02" and "ignore03". If you set them all to "ignore", an error happens.
Note too that the "set additional roles" dialog is a bit fiddly as it loses focus after each character is typed but it does work.