How can I disable utf8mb4 entirely on MySQL 5.5?
POLITICALLY CORRECT ANSWER
You cannot do MySQL Replication from a New Master to an Old Slave
I have written many posts about this:

Nov 26, 2012 : Replicate MySQL 5.0 Master to 5.5 Slave?
Feb 08, 2012 : will replication from 5.5.20 to 5.0.XX server work?
Dec 22, 2011 : What does the base64 BINLOG statements in mysqlbinlog output mean?
Feb 04, 2011 : MySQL master binlog corruption

In addition, there is the likelihood of dealing with the internal BINLOG magic number being in the wrong place and replication going senile. Here are my posts on the BINLOG magic number:

Oct 28, 2012 : Binlog has bad magic number
Dec 26, 2011 : mysql emits BINLOG rows even though binlog_&#173;format=&#173;STATEMENT

Since there is nothing in the meantime you can do, please upgrade your Slave to MySQL 5.5
ALTERNATE PERSPECTIVE
In all fairness, @ChristopherSchultz is correct in that you can implement it. I just yell and scream &quot;RISK !!!&quot; to be more cautious because of the BINLOG magic number's place in the MySQL Replication Universe.