I think you're supposed to put a * in the beginning of the password field, not in the beginning of the line itself. i.e
Assuming you want it in the password field, the following sed line will replace a * for all lines between 50-100:
sed -ie '50,100s/^\(.*\)\:x/\1:*x/' passwd.test
NOTE: Your file will be overwritten with the changes, so make sure you test it on a backup file first. Change 50,100 to whichever start and end line numbers your range of blocked users lie in.