First ask the webmaster if you can copy the forum - then, 
use reasonnable settings to mirror it (this will take more 
time, but it is better to let a mirror run several hours 
with reasonnable connection settings than clobbering the 
server bandwidth and being reported to your ISP as an 
abuser) :

Set options / Limits
Max transfer rate: 10000 B/s

Set options / Flow Contol
Number of connections: 1

Set options / Spider
Spider: No robots.txt rules

Set Options / Scan rules
+www.cieux.com/phorum-3.3.2c/read.php?*

AND MONITOR THE MIRROR to ensure that you don't go in 
infinite loops or other similar problems.