As long as you know the modems IP address then yes there are many ways.
(Note not a "Complete Shutdown" but modem's data is disabled)
Log into the modems Web-UI and go to execute a command
enter and execute this command:
modemcc 0 asy_add ?
Note the number returned for restoring WR31 modem!
then to disable the modem enter and execute these 3 commands (note it is assumed config 0 used at power up):
modemcc 0 asy_add 255
config 0 save
**NOTE** Once done you cannot restore remotely and must access WR-31 on LAN to restore! (see exception below)
To restore unit change the 255 back to original number noted earlier.
Remote restore can be done only if your service provider allows your WR-31 to send & receive SMS text messages, which depends on your service plans options.
If texting is available to the unit then setting up "TransPort router to accept CLI commands via SMS" will allow you to disable and re-enable via text.
See Digi Quick Note 18
Configuring a Digi TransPort router to accept CLI commands via SMS
Link to PDF: http://ftp1.digi.com/support/documentation/QN_018_Configuring%20a%20Digi%20TransPort%20router%20to.pdf
Actually Thank you for asking as I learned something new trying this out over SMS to my Digi WR-31