Why would you want to do that in the first place?
Yeah, the only way would be for the device to enumerate as a low-speed device only. I dont' think any hub can somehow "translate" speeds on the fly (thus making a connected device working at FS seen as a LS one by the host). I don't think any hub can otherwise make a connected device appear as a LS one only and make it function in LS mode only.
A USB device can declare itself as a LS device only to the host, but if it supports both FS and LS, I don't think there is a means for the host to require LS only. The host usually selects the highest speed both the host and the device support AFAIK.
But again, what you want to achieve is intriguing.