you need to have house rules...certain of things flatmates need to carry out and bear the bill. Eventhough in the contract indicate $120 include everything. You need to explain EVERYTHING means within the reason. If it's use abuse exceeded the capacity, they need to top up.
More, you gone back China last month, who else will abusively using the power and internet at home???
You can always check the usage details from your internet provider and check the power usage from your power company. Then give them the evidence.
THEY NEED TO LEARN. Free resources doesn't mean they can waste it away...