If you wish to withdraw money from your ISA, you will be able to do so by either selling loans to other investors or turning off reinvesting. Your funds will build in your holding account for you to transfer to your bank. Turning off relending is a more gradual process as you must wait for loans to be repaid. Once your loans leave the ISA wrapper, they will no longer be eligible for tax-free interest.
The Zopa IFISA is flexible, which means that if you withdraw funds from your ISA, you can replace those funds into the same ISA, within the same tax year. You can only replace funds up to the amount that you withdraw.