Take 'em to court to enforce the assessments. Let them defend on their thinking that they are awaiting foreclosure. The association must have, in its organizing agreement, that authority. It does by statute.
To take Mr. Brinkmeier's answer to the next step, Illinois condominium law allows you to take possession of the unit if the owner is delinquent, and once they are evicted, to rent the unit to pay the association back any amounts they lost due to the delinquency and attorney's fees. You need to bring the action in forcible detainer (eviction) court. The condo association needs to hire an attorney to do this as a member could not represent the association in court.
This answer does not constitute legal advice and does not and is not intended to create an attorney-client relationship. The law may vary depending on the state in which you reside. It is intended only to give some direction in which to seek assistance.