Unfortunately, I don't know how to disabe "New Defect" from Tools menu.
The only option would be remove permission on creating defects from a user group.
And to create your own button that would create new defect through TDCommand method.
But in this case you will not see standard Create new Defect Dialog box - so I don't think this is a good idea.
So probably it is time to adress hp support with this issue - maybe they know a way to hide/disable this option.