How to disable context menu on long pen press?

Caveat: I don't have a Bamboo, I've got a Tablet PC but what you're describing sounds like the right click press-and-hold action which you can disable/tweak as follows:

(if you're in category mode) Go to the Control Panel, 'Hardware and Sound' and then 'Change tablet pen settings'

(if you're in icon mode) or 'Pen and Touch'

Once inside, you'll see this dialog, and you want to change the settings on the press and hold action.

If you need to use the registry to alter this setting (Editing the Default Profile for example), it can be found in a DWORD key named TouchMode_hold, located in

HKCU\Software\Microsoft\Wisp\Tablet

Setting this value to 0 disables the behaviour.