Can I put a standard light switch in line with a 240v electric baseboard heater to cut power to it?
Yes, you can do that if the switch is a double pole switch rated for 240 volts or higher and is rated for the current of the heater. The National Electrical Code requires that the switch be rated for the load it serves. This means voltage AND current.
There are also line voltage thermostats with a positive OFF position so you wouldn't need the switch.
Whether you use a single pole or double pole switch it will work the same. It is a single phase circuit and it will work either way. However, the Code requires that the switch disconnect all ungrounded conductors. See below.
The difference is, with a double pole switch you are breaking the circuit in two places at the same time instead of one. This reduces the amount of pitting on the device contacts. With a double-break device the contacts will last longer at any given voltage. At 240 volts contact pitting is much higher than at 120 volts.
Additionally the National Electrical Code requires that all ungrounded conductors to fixed electric space heating be disconnected:

424.19 Disconnecting Means.  Means shall be provided to simultaneously disconnect the heater, motor controller(s), and supplementary overcurrent protective device(s) of all fixed electric space-heating equipment from all ungrounded conductors.
(C) Unit Switch(es) as Disconnecting Means.  A unit switch(es) with a marked “off” position that is part of a fixed heater and disconnects all ungrounded conductors shall be permitted as the disconnecting means required by this article where other means for disconnection are provided in the types of occupancies in 424.19(C)(1) through (C)(4).
...
(3) One-Family Dwellings.  In one-family dwellings, the service disconnecting means shall be permitted to be the other disconnecting means.

So, a 240 volt circuit would require a two pole switch if it used as a disconnect for the heater.
Good luck!