Amending lease to prohibit use of fireplace

For the first time in a decade, I have a tenant that wants to use the fireplace in one of my rental properties (SFH).

It is gas powered so there is no wood burning required, but the fireplace is in need of parts, has never been tested, nor has it been inspected for safe use.

I don't feel comfortable with the prospect of a tenant using a fireplace (wood burning or not) so I was thinking of amending the lease to prohibit the use of the fireplace. Is that possible? 

*The heater for the home works perfectly so the tenants are living comfortably. They are claiming it would be more cost efficient to use a fireplace than a heater (not sure if that's true in their area).