During Voyagers Season 4 and 5, the crew periodically encounter a group of waste managment freigters from a species called the Malon, who are in the business in dumping millions of tons of theta radiation into "isolated spaces".
During their first encounter, they offered the Controller of a freighter a way to reduce theta-radiation to a minimum the same way Starfleet does with their ships warp core.
This was declined due to disrupting the waste managment sector and "putting them out of business".
A few episodes later, in the episode Juggernaut, they encounter another Malon freighter with ruptured theta tanks, which explosions will wipe out anything in a three lightyear radius.
During a crawl through a service catwalk, B'Elanna and one of the Malon begin having idle conversation, much to B'Elannas dismay of course. In this conversation, the Malon describe their homeworld as a pristine, beautiful world, and to keep it that way the giant waste managment industry exists.
Wouldn't that be a perfect time to offer their plans to recycle Theta radiation again, which would keep their world pristine without the extremly hazardous and poisonous work? If not suggested by B'Elanna, why hasn't it been suggested by Janeway?