How do you tell someone that they smell bad and need to wash?

Be blunt about it. If it is something that really does bother you then you just need to tell them about. If they don't listen then either you threaten to kick them out or start looking for a new place depending on what your actual living arrangements are and which one of you actually own the house.

-Get them drunk
-Tie them to the hood of your car
-Run that fucker thru a carwash.

You use the term "my house": why are you letting this person live there? Offer him an ultimatum: be clean, including clothing and bedding, or move out.

Tell the stinky person to wash, or leave.

Tell them you’re going to kick them out if they don’t bathe.

Wait, hasn't washed in years?!
That's nasty...

Look them in the eye and loudly tell them "You fucking stink, go take a damn shower! The whole place reeks of your stench. It's like a bunch of hobos had an orgy in a dumpster full of piss and dead rat carcases"

Seriously, if I had to share a room, or even a house with someone like that I'd be very quick to speak my mind.

Move out and live by yourself and don't bother with them. That is the best thing for you. Don't try to fix someone who YOU think is broken. Just worry or concentrate on yourself.

Throw them out...

Tell them in a nice way again, but don't hold back on telling them that they really need to do something, of course, present it in a nice way; if they get bitchy with you then next time you notice they stink just be a dick about it and says something like "you smell like shit don't come near me." Then if they wash be really nice to them to reinforce positive behavior.

....for years....
...doesn’t he have fungus all over?

First step, u get water, al lot of water, second step u pour it over him while he’s asleep
:D
Scare him with close up pictures of those, diseases, bacteria’s and so on that gross him out
Make a bucket trap on top of the door

U could tell the landlord, he is probably messing up The whole house or flat with it anyways

Also ask that person why he doesn’t shower

I personally have a very sensitive nose, I would just go berserk seriously
U could also run around with perfume and deodorant in the whole house to counterattack and then leave, and I’m talking about baroque much, give him a huge headache.

You've lived with them for years and you can't bluntly tell them? What kind of relationship is this?

Simply say out load “who fucking stinks” every one will be come self concoction and check them selfs and or all look at who it is momentarily

Yo, stinky! Hose down!

Find a big lake. Take them fishing and playfully push them off the boat when you reach the middle of the lake. Throw them a bar of soap and refuse to let them in until they scrub.

Why is everyone assuming its a dude?

Pee on them during sex.