It looks like at the moment, only activities lasting more than one day are shown in the activity moderation tool (this page: https://www.bewelcome.org/admin/spam/activities)
If would be a good idea to show all activities regardless of their duration, since spam activities shorter than 1 day may also be created. Up to now it didn't happen, because I guess spammers prefer to make long-lasting activities to gain visibility. But in case a spammer decides to create a short one in order not to be noticed, we'll look dumb if we're not able to delete it.
Note: the kind of spam activities we typically have to delete are ones created for advertising dating sites.