Hi and welcome to Digi forum!
In order to block some specific traffic / sources, you would need to use the firewall. Please see the following guide that can be helpful in this case:
https://www.digi.com/resources/documentation/digidocs/90001019/default.htm#references/r_firewall.htm%3FTocPath%3DConfiguring%2520security%2520%7CFirewall%2520%7C_____0
https://ftp1.digi.com/support/documentation/AN_018_TransPort_Security_Lockdown.pdf
If any further help is needed for this, please send us an email to tech.support@digi.com and also review Support options here: https://www.digi.com/support
Thanks!
Anny
Digi Technical Support