Moved to Email Gateway for a better chance of getting an answer
you will need to start off by making a dictionary. go to email > dlp and dictionaries > compliance dictionaries. make a new dictionary and give the dictionary whatever name you would like. once this is completed, in the bottom section on the page, set the criteria to apply to the subject line. then click o nthe insert term button and enter whatever word you want to block and set it to wildcard. lets say you wanted to block the word 'purple' in the subject line. you would enter this for the term:
once the dictionary is made, you need to apply it to a policy at email > email policies. in whichever policy you want to use this dictionary, click on compliance in that policy. click 'create new rule.' give the rule whatever name you would like. on the next page, select the name of the dictionary you just created, then click next. on the next page asking what dictionaries need to be excluded, just click next. on the final page is how you want the gateway to act when the rule triggers. pick your desired action and then click finish. then click ok. apply the changes and that should take care of it.