It is possible, it really is. The trick is to have your regular massage lady. They don't poach each others' regulars. So "I'm sorry but I always use my friend Made 151" works for me. Of course if you become a regular at one particular beach spot they all know that "oh you are waiting for Made 151"
Of course there has to be a first time when you have to be firmish!!! Have your 40 or 50 thousand rupiah only in your hand and pick your lady wave the dosh and say "Just a massage and foot scrub please.OK"
of course she says yes. then you need to say "I don't want anything else during my massage. I am going to relax. Only you, no other ladies please, OK?"
At this point she knows that if she looks after you she maybe has a new regular and that you are paying a fair price and in my experience she will fend of all comers. I have heard they are more fierce at the Kuta end but this always had worked for me in Legian or Sanur.
Once you have a regular your problems are over unless she's at a ceremony when somebody will soon tell you and offer to deputise for her!!!!
Last tip, to avoid getting sandpapered, massage as soon as you hit the beach!!!!! No dip in the briny first!!! and hiring a beach lounge is just so much better than the old way of laying on your beach towel on the hot hot sand.