I am new to this whole OTRS System. So you could call me a bloody noob

My Problem is that I want to prevent the system from reopening already closed tickets. This happens when a customer makes a auto follow-up on a auto-respons mail from the system.
example:
#1 user creates ticket
#2 ticket will be handeled by agent
#3 ticket was closed successfully
#4 otrs sends eMail to user (msg: "Your ticket was closed successfully by fixing your problem")
#5 user has turned on a auto response feature of the mail client and sends a email (msg: "I am on vacation/ill and can not response to your email till XX.XX.XXXX")
#6 otrs can not handle this properly and will eropen the closed ticket because it interprets the user email as a follow-up.
As I said I am very new to this whole thing and I am not the one who set up the System. But now I am the only person who should admin otrs.
Is it possible to ignore every "follow-up" if the ticket is already closed and that's the optional part to send an email to the user with a msg: "Your ticket has been already closed for further problems on this issue open a new ticket. Thank you.".
I already found this topic that pretty much is what I want but I don't know what parameter to set to what value in the PostMaster.pm.
viewtopic.php?t=7361
We are using: OTRS 2.4.9 on a debian destribution.
In advance thanks for your help.
PS: Sorry for my English I am not native
