I believe this machine does not have a 3 way solenoid (but I could be wrong too) which releases the pressure back through the system when the pour is stopped, and therefore you still have the pressure built up when you unlock the portafilter - you will need to wait a few minutes to unlock it if you dont want the coffee spray. Or learn to very slowly release the pressure with a small turn and not let it explode out all at once.
The second as goes the mantra on this site - you need a better grinder (I know you dont want this answer but you cannot avoid the truth). The lower range tends to not be able to produce even size particles and you end up with lots of fines and lots of large particles all together and makes it much more difficult to get a nice even extraction at the right flow rate. The taste will significantly improve with a better grinder.
So the pressure left in the portafilter after the pour is most likely nothing to worry about. But the grinder is something to worry about - get a better one when you can afford it.