The Wemo needs to switch the DC control circuit. The Wemo can switch a relay with a line-volage coil (120v or 240v depending on where you are), and the relay contacts switch the boiler's DC circuit, possibly in place of the thermostat.
A safer way of doing this would be to have the Wemo switch a 5 volt wall-wart on and off, and use its 5v output to control a relay with a 5v coil. You can buy a 5v relay on a circuit board complete with terminal blocks on ebay cheaper than you can buy the parts so this route gives you a safe, easy to use solution at low cost.
I'll draw a sketch if you'd like.