I'm unable to reproduce this behavior. Lansweeper 5.3 shouldn't actually be capable of sending report alerts more than once a day. This option was only added in Lansweeper 6.0, which is currently in beta. If the reports were only sent a couple of times outside of the schedule, my first guess would be that someone manually clicked the Mail Selected Reports Now button under Configuration\Email Alerts.
Keep in mind as well that *event log* alerts are not sent based on a schedule, but as soon as the specified events are scanned. Perhaps the emails you're receiving are for your event alert, not your report alerts. If an event occurs multiple times, the accompanying event log alert will trigger multiple emails as well.