Stop subscribing over internet
Probably you have accepted an offer or subscribed for a spam Calendar accidentally. Watch the GIF below to know how to unsubscribe. Look for an unknown Calendar on the device under subscribed calendars.
iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware, unless you have intentionally downloaded spurious softwares or unauthorised apps directly from internet and installed on your device or/and have Jail Broken.