Those "weeds" are growing there because some soil has accumulated in those spaces and as long as there is some soil there you will have those "weeds" growing there. If it is not possible to lift up the blocks and clean the soil out, there are better, and less expensive, options than any glyphosate product. Vinegar can help some, but again growth will return unless the soil is cleaned out of the spaces. A torch could be used, with great care but agin unless the soil is cleaned out the "weeds" will return. Even boiling water could work.