Issue with module's hook_uninstall (not uninstalling db entries)
% is used as the placeholder marker for D6 queries, so you need to escape it with another %:

db_query("DELETE FROM {date_format_types} WHERE type LIKE 'eventdates_custom_date_%%'");


As a test I just added the following to the date_format_types table:



And then ran this query from the devel/php page:

db_query("DELETE FROM {date_format_types} WHERE type LIKE 'eventdates_custom_date_%%'");


And both rows were deleted: