In my experience, if you spend enough time with someone, and if you persistently resist any urge to act out your sexual feelings toward that person (and this includes fantasizing about them), then EVENTUALLY your feeling of sexual attraction will disappear, or at least subside to a manageable level. Then you will feel something like brother and sister, and you can be "just friends." But getting there may be a painful process.

It helps if (1) you have another sexual outlet (doesn't everyone?) and (2) the other person has unambiguously disavowed any sexual/romantic feelings for you. Any apparent ambiguity/ambivalence on their part will tend to feed your fantasies.

It works for me, now, because (1) I am married, and (2) the woman in question (not my wife) is a lesbian.

It also helps if you keep in mind that romantic love is a form of temporary insanity.