How do I turn off lending overall or lending just for loans?
To turn off lending entirely and go "lowercase", directions can be found from OCLC in How to become a non-supplier in the ILL Policies Directory?
To turn off lending for physical items, you can setup deflections, particularly for the RequestType "Loan". Directions are available from OCLC: Create Deflection Policies
Timothy Jackson from SUNY Library Shared Services has also developed some step by step directions: How do I turn off lending in ILLiad/OCLC?