You should be able to do that by cutting the trace that joins the 'digital' ground and analog ground islands on the top layer of BelaMini, see the illustration below:
You'll need a good knife (stanley knife?) and firm hand. Start with the tip of the knife immediately above the trace to cut and cut towards the bottom, so you won't risk damaging the traces that are above it. If you ever need to "cure" this cut, you'll need a jumper wire between the audio grounds and one of the other grounds, or scratch a bit more of the soldermask around the cut and put some solder on it to bridge it.
