As I've learned in the past, it's not the problem I have with these little darlings, its the problem they have with ME!
Cujo said it perfectly: tell HER you want HER to stop seeing YOUR friends. In a mental play way (alot of guys avoid this for lack of time to invest in this kind of Telenovela, or simply because their sacks are bigger than mine), its kind of fun to sit there and watch her expression. It takes a few seconds... wait for it ... wait for it ... BAM!
Funny thing is, *I* had my favorita tell me one day she would try to avoid all the paying John's who she thought were my close friends. The Chica Psychic Network is awesome and helps this become relatively easy. However a noble gesture, it's probably 95% lie and 5% honest intention. The problem is, it's STUPID, as she has a business, little bambinas to feed and all of that jazz. Stupid, stupid, stupid.
Back to the OP question, it's really my strategy in the last several trips down to start off the second session with the upfront, clear notice that future sessions do not include novia/novio classiciation, and instead FAVORITA becomes the word.
FAVORITA =/= NOVIA. Difference. Make sure THEY understand, and lather, rinse repeat as necessary. The more you monger in Latin America, the more you must put up your defenses against the very cunning ability of the women to mind-phuck you into submission.
The longer term players on the board and in this community simply do this now as matter of practice, and they protect any sillyness with the street smarts of big city pimp.