This might sound a bit
. But to me , as long as my horse has access to enough clean grass or hay and water then it is none of my business to what he decides to eat .
But then you have to take in the factors of how he is detecting the poisonous plants , if for instance you have just fed him his feed , and in his feed you have given him some supplements that may mess-up his taste / smell ( garlic for example ) then I would start to think about either waiting for a little while after he has eaten his dinner ( or maybe until he has eaten some hay or drank some water ) , so he can detect the wrong plants easily as they do naturally .
It is very easy for us humans , to start trying to control ( for the good of the horse ) what the he decides to eat.
But in the end , in the wild horses manage fine.
Bracken in very hard to control , but I think perseverance will get them down to a manageable level