You could try pushing the blade (with saw unplugged of course) against a scrap of wood held in a vice etc. Idea is to lock the blade from rotating. THen using a second block of wood as a cushion on the spanner, tap the second block with a hammer.
If you are using the original blade, then usually replacing it with a decent one like a Freud or a CMT will make a huge difference anyway. Watch the overall and hole diameters though.
For finer work, a larger number of teeth is normally better.
Other things that you can do is to put a second piece of material under the piece to be cut or to put some tape on the underside of the piece. THis should reduce or prevent tearout.
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