The exclusion script section allows for Ampscript to be added so records who return a true value ( a 1 in boolean terms ) are excluded from the send without needing to be added to a Suppression List.
Try this script, it will work:
Here's a little explanation for better understanding.
The 'Exclude_Data_Extension_Name' contains a field titled 'Email_Addr_Field_Name' and this field contains email address values. The Personalization String 'emailaddr' captures the subscribers email address who is being rendered for the send. This will be determined by your Default Email Address within the Journey. If your subscriber email address can be matched to an 'Email_addr_field_name' value in the 'Exclude_Data_Extension_Name' data extension, then a value of 1 is returned through the RowCount(LookUpRows()) functions.
Since 1 is greater than 0, the contact would return a value of true
and your contact would be excluded from the send.
P.S. Replace Exclusion_DATA_EXTENSION_NAME with your data extension name (which you want to exclude) and replace EMAIL_ADDR_FIELD_NAME with your Email column name/attribute. If you want to lookup against SubscriberKey, replace EMAILADDR with _subscriberkey