You can remove the donations option from your event page and future emails from Evite to guests about your event. On Evite's event creation page -- the one where you selected you invitation, entered the event date, etc -- go to the "Donation Options" section at lower right and click the "remove" link. See the image below.
People will no longer be able to make donations or contributions, but any funds already processed will be forwarded to the organization or beneficiary that you designated.
If you change your mind, or want to donate to a different organization, just go to the "Donation Options" section and select the charitable organization you'd like your guests to donate to.