How to stop MySQL replicating users on slave
For statement-based replication(the default for mixed binary logging format), --replicate-ignore-db=foo_db works only when foo_db is explicitly used as in USE foo_db. In a GRANT or REVOKE statement, USE mysql is not executed, as can be seen from the binary log.

For row-based replication, if --replicate-ignore-db=foo_db, any direct change to a table in foo_db would not be replicated to the slave. But MySQL always uses statement-based logging for statements that indirectly change the tables of mysql database(GRANT, REVOKE, SET PASSWORD, ALTER, DROP, CREATE), regardless of the value of binlog_format.

So, to stop replicating any changes in mysql database to slave, you can set binlog-ignore-db=mysql in your master's configuration file to stop logging any changes in mysql database. You can also temporarily turn off sql_log_bin for your session and CREATE/ALTER the user before turning the variable back on again.