I have come across walkers many times at cyb, marin and penmachno.
I very nearly rode into 2 pensioners sat on a deckchair on the last descent of the marin last year. When I stopped to tell them they were on a mountain bike route they told me to f@ck off.Then they generally became abusive and threatening. I lost the head, saw rage threatened to rip his head of etc.
Then realised hitting 70 year olds was not the best thing to do.
I called the excuse of a mountain bike ranger they had at the time (now replaced thank god)to report it and nothing more came of it.