I've tried to find this in the communty to no avail. My question is when you set an idea status as COMPLETED, what does that do? Is it searchable? Can people still vote and comment on it? The information bubble only says that it displays to users as being closed.
Nope. I thought about possiblily moving completed ideas to another area, but decided against it as I want all product ideas together.
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.