Disable the captive portal but keep the hotspot?

OK, I found a solution, not very elegant, but it solves the problem.

You just have to connect to Volumio via SSH.
Edit the /etc/hosts file
Add the following two lines:

127.0.0.1 captive.apple.com
127.0.0.1 connectivitycheck.gstatic.com www.google.com play.googleapis.com

Save, reboot and no more captive portal on Apple et Android devices.