Is it possible to remove the limit?
The limit increases as you rank up I believe. Nothing you can do about it without ranking up for the moment.
You don't necessarily need to report the post individually. You could indicate a comment saying "Please check the user's post history, where he mostly spam in bounty threads.". From there, you'll be saving a lot of time and your report won't be repeated on the moderator's end making both your job easier and without you worrying the report limitation.
This is helpful but likely most won't pay attention to it whom have lower ranks. They want the credit for reporting all vs getting 1 point for reporting a user.