I think you can use a well known antibiotic, with known concentration (this antibiotic can be considered 100% inhibitor) and compare the diameter of inhibition zone for your substance with that of known antibiotic)

This is a qualitative assay

You should culture the bacteria on Muller Hinton Agar in concentration equals to 0.5 Mac Farland standard

Then place the paper on the center of plate

After 24 h incubation the zone of inhibition should be measured