It is unlawful to erect a sign, or to leave a sign in place, on or near a public path that could lead the public to think that their rights to use the path were in some way diminished or absent.
So, if you have, for example, a bridleway that has a sign on it, or near to it so as to make you think the sign applies to it, stating "permissive footpath only", that is a misleading sign.
What can be done?
Make note of where the sign is, taking a photograph, if convenient.
Contact your local authority - possibly the Rights of Way section. Either write, pointing out the problem you perceive or make a telephone call. They will advise you on the best way forward.
Most local authorities have a statement of intent on their web sites and this may inform you further. Local authority staff are usually helpful but if you cannot get your problem resolved and you do not feel confident to tackle the issue alone, then seek help from a local RoW user group.