If the floating cells may be impacting your assay, you could try giving your culture an extra wash or two with media or PBS (gently).
It may be that these cells are sloughing off from your culture plate because of high confluency, or poor attachment, so another option might be to perform the scratch at slightly lower confluency, or to try a different coating on your cell culture plate/flask
What works best will depend greatly on the cell line though, as there is a lot of variability in adherence between human cell lines.