Go through your remote branches that contain some substring (or prefix) and prompt for deletion

clean_remotes.sh

#!/bin/bash
echo 'Clean up branches containing: '
read prefix

while read line; do
  branch=$(echo $line | sed 's/origin\///')
  echo "Delete this $line? [y/n]"
  #can't just read from stdin while inside loop
  read answer </dev/tty
  if [ "$answer" == "y" ]; then
    $(git push origin :$branch)
  else
    echo "skipped $line"
  fi 
done <<< "$(git branch -r | grep $prefix)"