I believe your event must defined using fully qualified schema and tablename:

In MySQL 5.1.6, any table referenced in an event's action statement must be fully qualified with the name of the schema in which it occurs (that is, as schema_name.table_name).

Reference: http://www.cs.duke.edu/csl/docs/mysql-refman/events.html#events-limitations-restrictions

So your query in the EVENT should be redefined (using Rolando's improved delete query):

DELETE FROM `ta_db`.`ta_table` WHERE ti_time < (NOW() - INTERVAL 2 HOUR);

I believe your event must be defined using fully qualified schema and tablename:

In MySQL 5.1.6, any table referenced in an event's action statement must be fully qualified with the name of the schema in which it occurs (that is, as schema_name.table_name).

Reference: http://www.cs.duke.edu/csl/docs/mysql-refman/events.html#events-limitations-restrictions

So your query in the EVENT should be redefined (using Rolando's improved delete query):

DELETE FROM `ta_db`.`ta_table` WHERE ti_time < (NOW() - INTERVAL 2 HOUR);