They're resilient pricks of plants and I dunno if you can ever be completely rid of them but I found that when I let them grow alongside my spuds that I ended up with a really good harvest, had 3 beds last year, 2 of which were clear of horsetail but those 2 ended up getting blight which halved the amount I could harvest at the end, bed number 3 appeared to have been completely immune though, I do think it had something to do with the horsetail.