Assuming that the remote for the fireplace is IR then your options are
- If the TV has an smartphone app you can use your phone to control it. That will operate via wifi not IR.
- Block the IR receiver on the front panel of the fireplace with opaque tape (black electrical tape should work. The fireplace remote won't work though.
- Get a custom installer to program a URC RF remote with RF receiver. The receiver would route the signal from the universal remote only to the device you intend to control. Not a cheap solution.