Don't know how to delete programs from solution / project

I've got about sixty programs in my solution / project.  (The entire solution is one project)

I decided that about ten programs were obsolete, no longer needed, so I deleted the source code (*.cbl).

But now when I try to build/rebuild either the solution or project, I get errors like these:

1>COBOL : error COBCH1502: Open fail : 'C:\PSMJ\TEST.cbl'
1>COBOL : error COBCH1502: Open fail : 'C:\PSMJ\X-BB-BENCH.cbl'
1>COBOL : error COBCH1502: Open fail : 'C:\PSMJ\X-FE-BENCH.cbl'
1>COBOL : error COBCH1502: Open fail : 'C:\PSMJ\X-FN-BENCH.cbl'
1>COBOL : error COBCH1502: Open fail : 'C:\PSMJ\X-MC-BENCH.cbl'

What can I do to make the solution / project completely forget about these programs?