It seems unlikely to achieve this requirement under the Self-sign portal. This is a self-registration process. As long as the corresponding information is provided, you can connect to the guest network. Even if the previous credentials are invalid, you can obtain a new valid credential through the portal again.
To eliminate this problem, you can only turn off the self-registration function, use the Sponsored Guest Portal instead, and provide network access credentials to guests through other means (such as API)