Does the auto-close option only take a positive integer for the number of hours? This makes sense, but there’s a (jobs) category in which I hope people will only be able to post topics in, with no replies. I tried inputting 0 to no avail.
This is not supported at the current time.
I like to implement the support for 0 hours setting. Added to my list with low priority.
We tried an “auto-close in 1 hour” category as a type of “no moderation queue category” work-around for a while. IIRC it worked fairly well (HAWK may remember better) but a zero-hour would have eliminated the rare few that got replies. The category didn’t work for us (an unrelated reason) but instant-close could be good for those forums that need it. 
It is possible to set it for minutes rather than hours using .n values
ie. 0.5
Yes, it is possible now.