How to disable H1 heading in CKeditor?
1) Take a look at the editors you're using on your site at /admin/config/content/formats

2) For each editor, you'll need to update the allowed tags.  Let's edit the FULL HTML format for this example, so click on the "Configure" button for "Full HTML"  (/admin/config/content/formats/manage/full_html).

3) Under "Enabled Filters", click the checkbox labelled "Limit allowed HTML tags and correct faulty HTML".  A filter settings form will appear below.  Remove the h1 tag from the field labelled "Allowed HTML Tags".



4) Save

Test that it's removed from the drop down: