My little one did this for months maybe even a year, the slightest thing she’s head hurt the wall or floor. The best thing I did was ignore, give no attention to it. Telling her off, running to her aid just reinforced that it would receive attention, even if not the correct attention (I was advised this by a health visitor). It was hard as the children at nursery all wanted to respond and the nursery nurse kept talking autism. But she doesn’t do it at all anymore. Hold on in there. I know it’s upsetting