If you’re alive then you’re totally be within your rights to choose who to voluntarily help or donate something to. Don’t like the look of that homeless guy for whatever reason? Don’t give them money. You can be as racist or misogynistic or otherwise generally cunty as you like, and as long as it’s your personal money/time/organs and you keep quiet about your selection criteria you’re unlikely to have a problem.
However once you’re dead, if you want your dickish restrictions honoured then you have to write them down somewhere. And any organisation set up to manage organ donations that agrees to facilitate such restrictions is likely to find themselves on the pointy end of a discrimination lawsuit at some point.