If you go into your notifications, there's a drop down arrow on the top right of their name - click that and you get the option to block them from seeing your tweets.
Sign in to your account and click on your followers. You will get list of your followers, click on the drop down button and tick the option block for the users whom you to block.
maybe the problem is me trying to do this on my ipad. Perhaps I have to do this on a PC or laptop. No drop downs on followers or notification screen, so will try on PC tomorrow. Thanks for help.