How to apply threshold within multiple rectangular bounding boxes in an image?

My question is that: I have ROI's for the bounding boxes around the objects in an image. The ROI's are obtained by the Faster R-CNN. Now what I want is to apply the thresholding to get the object accurately contained within the bounding box. The ROI of this image was got by the Faster RCNN.

So, After getting the ROI's, I only selected ROI from the image and pasted on the black image of the same size and dimension which result in the following image.let say

As you can see that boxes are rectangular so in some places it covers some background area along with spikes. So, how can I apply thresholding to get only the spikes and other pixels turn to black?

from How to apply threshold within multiple rectangular bounding boxes in an image?