To effectively block VPN applications and browser-based VPNs on your NGAF 5300-I firewall, ensure that you've correctly identified and added the necessary application signatures associated with these VPN services in your denial rule. Additionally, consider implementing SSL decryption to inspect encrypted traffic, as some VPNs may use SSL/TLS for obfuscation. Update your firewall signatures regularly to stay current with emerging VPN services. Furthermore, make sure that the denial rule is placed at a higher priority in your rule set, allowing it to take precedence over other rules. Finally, monitor logs and adjust the rule as needed to maintain effective VPN blocking.

Check if the rule is set to block both incoming and outgoing VPN connections, and make sure it's applied to the right interfaces or zones where the VPN traffic is passing through.