The easiest way to do this online would be to revoke all
REPLICATION SLAVE privileges on the master for any users who have it.
This offers the additional benefit of allowing you to be selective about which slaves you remove.
Once that's done, check for any existing replication sessions with
SHOW PROCESSLIST and kill them off with the
KILL command (these might drop off by themselves after you revoke the privileges, but I suspect this isn't the case).
You need to have in mind that, using this approach, the replication won't completely stop on slave, what will happen is that the
SLAVE IO_THREAD won't be able to get new statements (queries) from master, but if you eventually have queries that you already get from master(these are saved on your relay log) but didn't execute yet (you can check if you have seconds behind master to see if it is the case), it won't prevent these queries to don't execute.