Get lock out plates that fit over your new circuit breakers or just a big inline cut off switch that you can lock off.
http://www.homedepot.com/p/GE-30-amp-240-Volt-Fusible-Outdoor-General-Duty-Safety-Switch-TG4321R/202978635?N=bm0h#
No problem as long as the double pole breaker is the same rating as the single pole breaker. 20 amp 240 volt substitute for 20 amp 120 volt. Just use one side of the breaker. Remember this is temporary and needs to be corrected.