It depends what you mean by “disarm” and what you mean by “the alarm.“
There is no way to change the smart home monitor “armed state“ unless you’re connected to the Internet. So whatever local automation you have set up to run in the armed state will continue to run. If you have a rule that says “if the armed state is away and the front door is opened, turn on all the lights in the house and set off the siren,” Then if the armed state is away and You lose connection to the smart pthings cloud, then every time you open the front door, all the lights in the house will turn on and the siren will sound until you get access to the smartthings cloud again. So in that sense there is no way to disarm the smart home manager at all without access to the smartthings cloud.
And you couldn’t use the smartthings app to either change the rule or turn off the siren because those things require the cloud as well.
But you could certainly cut power to a plug-in siren with a locally operating switch , just like you could take the batteries out of a battery powered one. Or just unplug it.
Then if you open the door the siren would not go off because it didn’t have any power. The Smart home monitor armed state would still be armed, and the rule would still be in effect, and the hub would still send a message to try to make the siren sound. But without power, no sound would be made.
Of course, then you would also have disabled your security device.
But that’s the only option if you don’t have access to the smartthings cloud, to literally take the individual devices off power altogether so that they could not be turned on from the system again until power was restored.