Password protect the VBA project.  Open the VB Editor and choose Tools |
VBAProject Properties, then on the [Protection Tab] check the box next to
"Lock Project for viewing" and enter and confirm a password.  Be sure to
write down the password somewhere safe - these are tougher to crack if you
forget than passwords for workbooks/worksheets.  Or save a copy without the
password protection for your own future maintenance needs.

After you do this, when you reopen the workbook after closing and attempt to
open the VB Editor, you'll be prompted to provide the password before you can
even see the code.