Hello, I'm wondering what's the best way to avoid the unpermitted
unprotecting of worksheets in excel...

I know that the starting point would be to simply protect the sheet with
password, but it's easily crackable with tons of free soft on net. What
additional layers of security would you propose in order to complicate the
task of some malicious individuals...?

In particular, I know that in our office there is somebody who knows the
add-in published by Einar Ståle Huse on straxx.com... Is there any
possibility to block (by default) the run of any "non standard" add-ins on
the file I create? Or would you propose to include a VBA (pass protected)
that would overrun any cracking attempt (but for this to happen, the person
should enable macros, shouldn't it?)...

Or could I at least make a VeryHidden "History" (again, with VBA pass
protected) sheet that would at least keep track of any intrusions and bring
proof of it?

Hint: On the machines in our office, you can include add-ins, but not
install programs...

As I told, I don't have illusions that you are able to totally block a
determined individual from cracking the protections in excel, but I would
like to complicate this task as much as possible... (ideally by putting in
place a protection that is impossible to crack without INSTALLING a specific
PAYABLE software)...

Many thanks for your hints/solutions!
Regards,

Mark