Actually t-mobile does let you do some stuff. It's not widely known but if
you go to the my.t-mobile.com page (under communication tools) it lets you
do many things. Here is a snippet from the web page

"TEXT MESSAGING FILTERS AND SPAM BLOCKING

Gain greater control over how e-mails sent to your phone should be handled.
The tool below will allow you to implement filters that can look for emails
containing certain words, from specified senders, or other criteria, and
handle them in a way that you select. The filters that you select will also
appear on this page, where you can delete or edit them as you choose.

Block all messages from E-Mail to handset  (yes or no)  * (If you select
yes, you will NOT receive any E-Mail messages to <phonenumber>@tmomail.net
or <alias>@tmomail.net)

Allow messages ONLY to "My Phone's E-Mail address"  (yes or no) *
<alias>@tmomail.net (Recommended)

(If you select yes, you will only receive E-Mail messages to the alias or
<phone's E-Mail address>@tmomail.net) "

alias means you can have dansphone@tmomail.net instead of
6305555555@tmomail.net.
good for giving out your sms email w/o your actual phone number.

No it's regular text messages that I want to stop--but only from peolpe whom
I don't know.

Actually email messages can't be turned off either. They are either on or
off.
There is no way to select which users can sent email either, the black
list/white list feature on Cingular's web site doesn't work nor does the
feature that they claim will allow only email to come through that is sent
to your alias not your regular phone number.