In case anybody wants to try this. I haven't tested it, but it seems plausible that it could work to disable GPRS/UMTS while still allowing calls, SMS, and obviously WLAN.

1. Edit, as root, /etc/default/csd-gprs
2. Change ENABLED=1 to ENABLED=0
3. Save and exit
4. (as root) stop csd
5. (as root) start csd

CSD is the "cellular services daemon". I assume that disabling the csd-gprs plugin GPRS should stop working.

TRY AT YOUR OWN RISK!