Within posts, comments and polls you can turn the comment field off. If someone has already commented, the comment will still be visible to the whole group and it will say "comments disabled" below. You can change this back the way you disabled them. The comment field is by default on.
Press "Edit" > "Comments" and choose "Disabled" You can do this both for single events or a whole series at the same time.
Posts and polls
Press the downwards arrow in the post then untick "allow comments"