There is an offline in the GUI, but if you don't want to risk screwing anything up, I wouldn't use the GUI ;) (sorry William)
I've seen too many disk replacements gone wrong lately...
Look above in post #23.
# zpool status -v pool: DEC2010 state: DEGRADED status: One or more devices is currently being resilvered. The pool will continue to function, possibly in a degraded state. action: Wait for the resilver to complete. scrub: resilver in progress for 0h3m, 0.36% done, 13h59m to go config: NAME STATE READ WRITE CKSUM DEC2010 DEGRADED 0 0 0 raidz1 DEGRADED 0 0 0 gptid/218a2962-c5d4-11e0-8fd1-0025900d25b0 ONLINE 0 0 0 gpt/disk1 ONLINE 0 0 0 gpt/disk2 ONLINE 0 0 0 replacing DEGRADED 0 0 0 5817244552391206561 OFFLINE 0 0 0 was /dev/gpt/disk3 ada3p2 ONLINE 0 0 0 5.17G resilvered gpt/disk4 ONLINE 0 0 0 gpt/disk5 ONLINE 0 0 0 errors: No known data errors
# zpool status -v pool: DEC2010 state: DEGRADED scrub: resilver completed after 12h20m with 0 errors on Fri Apr 13 04:52:20 2012 config: NAME STATE READ WRITE CKSUM DEC2010 DEGRADED 0 0 0 raidz1 DEGRADED 0 0 0 gptid/218a2962-c5d4-11e0-8fd1-0025900d25b0 ONLINE 0 0 0 gpt/disk1 ONLINE 0 0 0 gpt/disk2 ONLINE 0 0 0 replacing DEGRADED 0 0 0 5817244552391206561 OFFLINE 0 0 0 was /dev/gpt/disk3 ada3p2 ONLINE 0 0 0 1.39T resilvered gpt/disk4 ONLINE 0 0 0 gpt/disk5 ONLINE 0 0 0 errors: No known data errors