Backdoor routine - the sequel

In a previous post I described a problem I was having with a backdoor routine after disabling access to the VBE via either Tools/Macros/VBE or ALt+F11 - thanks to faster, Ivan, and Jerid for help on that one.

I've now found that if another workbook is opened first and access gained to the VBE before the workbook containing the backdoor routine is opened, the VBE remains available. (And yes, I have password protected the project with a password greater than 8 characters. I just want to make it as difficult as possible for anybody who may try to amend the code.)

Is there a way for to test if the VBE is already open when a workbook is opened and, if it is, to close it?