I'm pretty sure that it can only be done on jailbroken phones.
Put this launchd plist in /System/Library/LaunchDaemons. It will trigger the script whenever the sms database changes.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.billybob.SMSremote</string>
<key>ProgramArguments</key>
<array>
<string>/usr/sbin/script</string>
</array>
<key>Nice</key>
<integer>20</integer>
<key>WatchPaths</key>
<array>
<string>/private/var/mobile/Library/SMS/sms.db</string>
</array>
</dict>
</plist>
For the script I'd use something like the following to determine if there is a message containing the string:
sqlite3 /var/mobile/Library/SMS/sms.db "select 'String Found' from message where text like '&&XX&&' order by date desc limit 1"
for the whole script maybe
case $( sqlite3 /var/mobile/Library/SMS/sms.db "select 'String Found' from message where text like '&&XX&&' order by date desc limit 1" ) in 'String Found') sqlite3 /var/mobile/Library/SMS/sms.db "delete * from message where text like '&&XX&&'" ; commandscript;;esac
In order words when the string is found, delete all messages containing the string and execute the commandscript.
Of course you need a jailbroken phone and sqlite from cydia. The same process could be done on the other databases as well. I'm not sure how you would go about doing this without a shell script but I'm sure it's possible. I haven't tested the script yet so you might want to make a copy of your sms.db before trying.