While looking through the code in regards to the vnum and roomflags configs, I noticed that while there's no display for it the config list, you can still config + or -nice. I'm assuming this is an oversight -- Judging from the only instances of PLR_NICE I see in the code, my assumption is that the original coders decided to make PLR_NICE more of a punishment than anything, or perhaps meant to take it out altogether. Assuming the former rather than the latter, here's the very simple fix.
In act_info.c, in function do_config, find and remove:
else if( !str_prefix( arg + 1, "nice" ) )
bit = PLR_NICE;
If nothing else, this will at least ensure that players can't config it themselves.