Most servers do this buy utilising the pb_sv_task function on punkbuster.
You need to access the files on your server either through FTP or a server control panel like TCAdmin. Once you've got access you need to open the pb/ folder, if you're streaming to PBBans you should already have the pbsv.cfg file in this folder. If not create the file yourself using notepad, then save as .cfg .
Once you've done this you can start to enter the commands into the config, it is done in this format. Both times are in seconds.
pb_sv_task <time_it_is_first_run> <How_often_it_will_repeat> "say Type your message here"
For example, I would use something like this on a server.
pb_sv_task 0 60 "say This is a test message"
You then add more lines to add more messages. For example,
pb_sv_task 0 60 "say This is a test message"
pb_sv_task 30 60 "say This is a second test message"
Hope this helps,