The "time police" or "temporal enforcers" or whatever they were called was a bad idea from the get-go. It was a concept that let the genie out of the bottle. It would have made a good science fiction story but it's not a good Star Trek story; Trek has always depended on time-travel stories.
If the time cops were really able to do their job, everything after "The Naked Time" would be an alternate timeline. They would have stopped the Enterprise from travelling back three days, thereby changing everything that came after.
It's like the TNG episode where they decided that the transporter retained a record of everyone who had been beamed out, and they could "restore" that person to their previous condition. Well, that certainly would come in handy, wouldn't it?
Likewise, any time Nero, or Rasmussen, or Future Guy or any other time-travelling trouble maker showed up, all our heroes would have to do is fold their arms and say "Not our problem! The Temporal Police will be here any second to fix everything."