How to avoid captivate to convert ppt slides to swf?

Only FMR slides use SWF-based technology but with output to HTML5 those slides will be converted to MP4. It is not always working well. I recommend, when such a Full Motion Recording slide cannot be avoided (most can using an alternative) to stop the recording and insert a Video Demo slide recording. Those slides are natively exporting to MP4 and moreover have a better quality and editing possibilities. That for a software simulation.

In your case for PPT imported slides, even though they 'look' as if SWF technology is used, HTML output will be OK because that is fixed on publishing. However PPT is meant for presentations, not for eLearning. It would be much better to unzip the PPT, import all the assets to a Project Library in a new CP project and start from scratch, using PPT as sort of an initial storyboard. You will quickly find better ways to improve learning and not be restricted to the movie slides which are created when importing PPT where none of the individual objects can be controlled.