During the cycle for creating proposals, go to Proposals -> New Proposal -> Remove lambda:
From the dropdown, select one of the lambdas that are currently available in the contract. These can be any of the functions included at contract origination or subsequently added. When you select the lambda, the field below will be populated with its respective Michelson code.
Click Submit and approve the transaction. This creates a proposal, that, if accepted, will allow removal of the selected lambda at execution.