It's doubtful that Apple would be offering you 50 GB free storage and doing it by creating multiple calendar events. It's also fairly unlikely you have a virus on your iPhone or affecting your iCloud account. If you are using the Mail app on your iPhone or iPad (or maybe your Mac - I don't have one of those) to retrieve email from a non-Apple account, that is the most likely culprit. It could well be that your other account has been compromised and that's where the calendar events are coming from.
First thing to do is change your password on all the email accounts you are logged into on your phone or use the Mail app with. Change your iCloud password too, mainly for good measure. Then you can delete all the extraneous calendar events and hopefully they won't come back.
You can check how much of your iCloud space you've used by logging in to icloud.com and scrolling to the bottom of the page. You'll see your usage and your allotment. 5GB is the free account. Next step up is 50 GB for $0.99/month, which is a very good deal. 5 GB isn't really enough these days. 50 GB is just right for me, especially at $12/year.
Note: You can also check your iCloud usage in the Settings menu of your phone. Settings -> [Your Name] -> iCloud.