Electrically you could replace all the sockets with ones with 1A circuit breakers. That limits them to 250 watts or so per socket. Supply a hard-wired kettle and hairdryer.
However the temptation will then be to over-ride the circuit breakers somehow. Getting bayonet adapters off ebay and running the heaters off the lighting circuit ....
If you can't manage the tenants by frequent room inspections and enforcing a no-heaters rule, you need to bill them.
Why are they using portable heaters? Do you not provide central heating? You could provide fixed heaters on time switches, press for 1 hour heat. At least that would stop them leaving heaters on continuously when they go out.
I have a feeling that the OP is renting out a ****hole.
