TL;DR: Real life yandere might be decent. if you don't want one, first reject in a neutral way. Doesn't work? Restraining order or other measure taken by authorities.
Depends completely on the personality though. Actually, a lot of real life yandere might be better and safer to date than their 2D counterparts.
Think of it: 2D yandere are often more stereotypes than actual individuals, and therefor display more over the top violent, aggressive and "yandere" behavior. Take a commonly known yandere, Gasai Yuno, for example. Yuno snaps at everything someone does to Yukiteru, even if it is only looking at him. Mirai Nikki, while the story mostly takes a serious tone, might almost be a parody on yandere. If Yuno got any more yandere, she would be killing everyone who mentioned his name...
The point I'm trying to make is that a lot of 2D yandere, and with them peoples perceptions of yandere in general are exaggerated by parodies and comical stereotypes.
I met a couple of people who have yandere tendencies in real life (which you are free not to believe, as I am posting as an anon here and could've made this all up). Those persons didn't carry around huge cleavers, katana or guns around on the street. Often they where not even that violent. The yandere traits all of them DID have in common however, is a way stronger and extremer sense of love, faith and dedication. A "real world" yandere might actually be very safe and nice to date, if you can actually show her (or him) love and trust. No two-timing, no flirting with others, no sitting behind your desk not giving her attention. Then again, if you do any of those why would you have a relation in the first place...
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