Negative keywords are the best friend of a PPC Marketer. Deciding who is your target audience is as important as deciding who is not your target. With a comprehensive negative keywords strategy, you can
Improve ROI,
Save on the cost,
Reduce wasted spend and,
Laser target your campaign.
Read this post to learn
How to build a negative keyword list
Where to apply negative keywords (Account Level, Campaign Level and Ad group level)