If they do show interest when the new milk comes, you could fit the Quiet Wean nose tags. It doesn't harm or hurt the mother (or the stirk) but does prevent them getting a drink, so they soon stop trying. I used them on year-old weaned Jersey x and Hereford x stirks when I put them back with their mother, who was still being milked.
You can buy them singly from the UK agent, although when you Google it looks as though you have to buy a packet of 10. If you want some and can't find where to get them in small quantities, shout and I'll dig out the details.