MySQL Forums

Ok, i'v found it deep in the documentation PDF.

Here goes to anyone who needs the same "feature".

Quoting:
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read_only
When the variable is set to ON for a replication slave server, it causes the slave
to allow no updates except from slave threads or from users with the SUPER
privilege. This can be useful to ensure that a slave server accepts no updates
from clients. This variable was added in MySQL 4.0.14.
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