RE: People with negative reputation or more number of downvotes should be restricted from doing "vote" and "comment" operations maybe even "transfers"

"Would it be a good idea to restrict people from broadcasting transactions if their account reputation goes below a certain value?"

NO.

I'd rather limit their post/reply activities to one for 24 hours without setting any limitations on transfers. This would limit their annoyance to the community but not lock their funds or stop them from using the platform.