im a company here and providing for 10 years . have blacklisting experience since then and even know two staffs of Tea*Speak support team.
there may be some tricks to delay it but avoiding 100% is not gonna happen
its not related to firewall. firewall do two things. block,allow. nothing between them
you have some issue in your server .maybe lack of resources or network issue
in 1.5.6 that codec related bug has been fixed. i personaly contaced @WolverinDEV and we tested it . its not there anymore
your issue is something else. maybe some ddos you facing there
its the channel bug used by some guys in back days.
its related to channel creation with invalid voice codec
disable your temporary channel creation and any way to edit channels by anyone in your server
or simply upgrade to 1.5.6