Spoke to one of our engineers and he provided the following. I hope it's helpful in what you're trying to do.
There is no way to close comments on multiple Idea's (bulk), however there is a UI and REST API to close comments on a single Idea. As a customization they could potentially create a custom REST Endpoint that iterates over a set of messages and calls the REST call to close comments emulating a batch (bulk) operation.
Via the UI
Options Menu>>Block New Comments
See this REST doc for (a little) more detail.
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