Well it has taken a long time to rid my mare of hers, particularly due to the foot problems she has.
My farrier has continually trimmed away at it. I have given salt water foot baths and sprayed with iodine, and finally after a very long time the seedy toe has virtually gone
As a donkey owner this is something that I have had to deal with a lot. Your farrier needs to cut away the area and open it up to the air (it is anaerobic). Iodine is excellent and scientifically proven to work in a sugardine paste, but Antibac from your farrier is now known to be the best treatment. It costs £15 and lasts a long time, is easy and effective to use.
My old school farrier always recommended live honey brushed into holes/gaps/white line etc.
Worked with my boy and everyone else who's tried it (I think)!
I used sheep foot rot spray after farrier pared it a bit. That worked. Needed to keep the foot regularly trimmed as it recurred if her horn got a bit long.
Need to ask why it is happening in the first place. Is there any WLD too?
Can be very stubborn - I clean all the soft stuff out - then I use a gel called White Lightening and plug the hole with cotton. Needs to be cleaned out, dressed and re-plugged every day for a couple of months really. Hard to do with a shoe on.
There was some suggestion that copper supplementaiton helped wiht seedy toe. At the moment I'm supplementing additional copper so I'll see in a couple of months time if that has had an effect.