MySQL how to release memory used up by SELECT
You could run RESET QUERY CACHE to eliminate collected result sets in the query cache. You could also be more aggressive and drop the query cache and recreate it a different size. For example, to set the query_cache_size to 512M, do the following

SET GLOBAL query_cache_size = 0;
SELECT SLEEP(30);
SET GLOBAL query_cache_size = 1024 * 1024 * 512;


Unfortunately, you can't release the MyISAM Key Buffer and InnoDB Buffer Pool in the same manner. You would have to resize them in /etc/my.cnf (or my.ini for Windows) and restart mysql.

Reading your comments, I can confidently say that MyISAM does not cache data pages. The query cache is the only thing I can think of that grabs and holds result sets. If you are loading in bulk, there should be a lot of query cache invalidations going on.

I am not Cluster Certified, but I will take shot at saying this: Since you are using MySQL Cluster, you may want to try shutting down any mysqld processes or having the cluster disconnect data nodes. Also, be aware of the transmission speed that data hops around the Cluster.