Syntax:
{DEALLOCATE | DROP} PREPARE stmt_name

To deallocate a prepared statement produced with PREPARE, use a
DEALLOCATE PREPARE statement that refers to the prepared statement
name. Attempting to execute a prepared statement after deallocating it
results in an error. If too many prepared statements are created and
not deallocated by either the DEALLOCATE PREPARE statement or the end
of the session, you might encounter the upper limit enforced by the
max_prepared_stmt_count system variable.

URL: https://dev.mysql.com/doc/refman/5.7/en/deallocate-prepare.html

