Sonunda onunlar gittiler! Finally they went! (referring to pests in the house!)

Thanks

Hehe-I thought you were talking about me.

Anyway I think if you use the plural onunlar you don´t have to use the plural gittiler.but just gitti

Maybe someone can say if this is correct or not.

Me too - I thought it was like ´Sonunda has left the building!!!´  

I think you are right and possibly can just use gittiler on it´s own - but why ´onunlar´ and not ´onlar´? 

My try

En sonunda gittiler.

Why do you need ´en´?  The word ´sonunda´ on it´s own means ´finally/ultimately´. 

"en sonunda" sounds better to me  . Cant explain why...

Or shall we use " yok etmek" instead of "gitmek" ?

I would say: nihayet defolup gittiler!

that´ true but it is not "onunlar" but "onlar"

But actually I would skip using the personal pronoun here and I would say "sonunda gittiler" or "en sonunda gittiler". 

en just makes it stronger.

you could say sonunda yok oldular

or even

sonunda yok olup gittiler

or sonunda onları yok ettim (if you made them disappear)