I am not aware of a plugin at the moment.  You may need to do some special coding.  I would recommend if you do the special coding to set it up to do the replacement immediately upon user input, that way you catch it right away.    Having the system constantly scan the whole site for words is server labor intensive and you dont want that.  Its better to do it right when they enter something.

Also be aware that major credit card companies do have auditors that check sites out from time to time and if they do find that the site is attempting in any way to sidestep policies it could mean trouble, so just be very careful and adhere fully to policy and you will be just fine.  Understand that they do this because they too are liable and its a huge legal issue for them, just be careful..

Just thought i might pass along some good info for you.   There are Oxwall coders available if you need to hire someone... :)  you can find them here http://www.oxwall.org/market/specialists

Thanks for the advice, yeah they mentioned they would scan the site - which is why I wanted to change the words before they're inserted to the database. That way no one has really broken any rules, as the words were never present in the first place.

Humm, I'm thinking some kind of checking needs to be done, on the POST method before the comment or whatever it is is inserted to the database?

Thanks for the link, I will check it out!