I have removed your attachment, as it only contained an executable file - which is of no use to us in terms of answering your question. For the safety of our members, we do not allow executable files.
Despite the rumours and claims, VB programs cannot be decompiled. For more information, see the article
How can I get the source code of a compiled VB file (.exe/.dll/..)? from our
Classic VB FAQs (in the FAQ forum, which is shown near the top of our home page)
Any program can be debugged or disassembled, and there are various other ways to determine what the program does too.
Without details of what happened, we cannot tell what things were done in this case, and therefore can't tell you how to protect against it.