Even if they consented to such conditions in the past, if they have since changed their mind and wish to remove consent, how is it ethical to not allow them to do so?
There are very few situations where continuous consent is important, and I don't think "releasing documents" is one of them.
Informed consent is one of the bedrock principles of (ethical) human subjects research. Part of the process is that subjects have to be able to withdraw from the study at any time.
In the US, this is literally the law (45 CFR 46.116), which says that "the subject may discontinue participation at any time without penalty or loss of benefits to which the subject is otherwise entitled." This would likely include the release of protected health information after it is collected, especially when it comes from a small probably-easy-to-deanonymize group.