Should I decline unrelated work assigned by PhD advisor?
Very politely notify them your scholarship and RA position (or whatever it is called that requires you to do work in return for money) has ended, and offer a consulting rate to continue. Make sure there's an agreed-upon number of hours for each task, so they aren't thinking they can give you one hour of work at a time, when the emails alone take longer than that. Research in most fields doesn't happen unless there is a fountain of money behind it, so this is not as beyond the pale as it may seem from your perspective. Just be careful to make clear it is simply the economic situation of the real world (which apparently they forgot they live in) that is the issue, not that you are making these demands to be unhelpful and make them go away.

If they can't pay you, then you can offer to help someone else take over the work, for example by giving them a list of steps and answering their questions via email.

Or you can offer to help in exchange for co-authorship, but then make sure you get into the loop so it is clear to all that they are violating journal terms by not including you as a co-author. If they are dependent on your analysis for their paper at this point, they may put in a position where you are compelled by professionalism to help them finish, but of course this needs to be out in the open with a promise of at least co-authorship in return. 

Everyone with valuable skills has to deal with this kind of thing. People will push until you push back. The overwhelming majority aren't necessarily as cutthroat as the occasional bad joke might imply; they just convinced themselves your time is near-worthless while theirs is precious. Clarify to them this is not the case anymore.