I'm not sure of thread necromancy here, but I found the solution to this one finally.
The _a_*.xtbl files in the misc_tables and da_tables vpp packages are.. ignored. I don't know why. Maybe it's a holdover from SR2 when they didn't have specific spots to activate specific levels of the activity?
Anyways, the files you want are in the _a_ACT_LOC_LVL_modal.str2_pc file in sr3_city_0.vpp_pc. As in for say trail blazing northwest region level 2, it's _a_tb_nw_02_modal.str2_pc (Genki is rm for Running Man). Inside the str2 file is another copy of the given xtbl file. Modify that file, repack the str2, and slot it into the game root directory to override it. It should work.
Note that I had a lot of hassle, it may not work if the file size changes, or it may just have to re-index it or something or be really slow. I can attest however when it works it actually works and lets you modify the stuff given in the xtbl for that given activity like time, requirements, reward cash/respect, and so on.
The actual global tunings for all activities are in activity*.xtbl in misc_tables.vpp_pc, those control things for all the activities of a type as opposed to that specific one like the _a_*.str2_pc ones do. Those files can be dropped into the root folder and will override stuff as needed.
Oddly, if you have say turn off spawning in the disables flag, you ALSO need to turn off distant spawning and roadblocks otherwise once you move you will see vehicles/peds. Parked cars is independent from those also. Without checking SR2's design I suspect the disable spawning flag is for say fight club to ensure nobody pops up in the middle of where you are, and doesn't automatically turn off distant spawning since you can't move from the cage while the activity is in progress thus it's not needed.