Ratio can't be negative, but ROI can. Positive ROI is the percentage of the ratio being 1 or higher. Negative ROI is the percentage of the ratio being less than 1.
Or, from a "common sense" perspective, if I spent $1000 on advertising, and earned $100 in revenue from that investment, my ROI is definitely negative.
Or, from a "common sense" perspective, if I spent $1000 on advertising, and earned $100 in revenue from that investment, my ROI is definitely negative.