Nizo S56 Problem

[PhiPsi]

Hello,
a Nizo S56 found its way into my belongings. It didn't cost me a penny and it seems to work. Actually, it works too well, it refuses to switch off, when I let go the release. Even when I turn off the main switch, it continues running. The only way to stop it, is to disconnect the battery compartment.
Is this a common problem? And does anybody know, how to repair it?
Thanks,
Philipp