You can delete up to 500 at a time with Hive Manager Classic. If you go to Monitor> Select the alarms for the AP in question> At the top of the page near the total count of alarms, you should see "Items per page". You can set this up to 500, which will be faster than deleting them 100 at a time. However there is not a way to delete several thousand alarms all at once.
In that case 100 will be the fastest way to get through those. You can request that we add an option to do all at one time or in greater numbers by using the feedback feature in the small side bar on the right hand side of each page in NG. The feedback option in the middle option with the icon that looks like a chat bubble. This will send your feedback and request to our engineering team for them to review and add your requested features to future NG updates.