There are ways to combat spam that still allow for anonymity.
One example is proof-of-work. You can require a submission do 3-5 seconds worth of work on typical high end laptops/phones so that it becomes cost prohibitive for someone to post thousands of submissions an hour. There are ASIC resistant algorithms to prevent spam-for-hire services that might try to game the system.
Most users are passive consumers of content anyway so they wouldn't notice this process and when they want to post a few times per hour or day then the 5 seconds lag will be worth it to have a better social experience.
One example is proof-of-work. You can require a submission do 3-5 seconds worth of work on typical high end laptops/phones so that it becomes cost prohibitive for someone to post thousands of submissions an hour. There are ASIC resistant algorithms to prevent spam-for-hire services that might try to game the system.
Most users are passive consumers of content anyway so they wouldn't notice this process and when they want to post a few times per hour or day then the 5 seconds lag will be worth it to have a better social experience.