I'm sorry to say that, because this is something that I do not care to do, I really do not know the answer, or if it is possible using a method other than usin the third party app Tasker.
However, if it is possible to set the default ringtone for text messages to "silent" on the eris, I know that you can set specific ringtones for individual contacts, and that may be the answer. However, as I said, I don't recall if the eris can be set to a silent ringtone by default.
Another idea is to find an mp3 that is just a few seconds of silence and use that as the default text message ringtone (by copying the file to a folder called "notifications" on your sd card.)
But, again, it is a complicated app, but this is something that Tasker can do.