Before pro-tempore moderators are appointed, SE employees usually keep an eye on the site and deal with flags. This typically takes a bit more time than when you have your own mods, but usually they check the flag queue at least once a day. This is not necessarily true on weekends or holidays, though.
Flagging as spam is the correct action in any case as already mentioned in the comments. It will draw the attention of any SE employee visiting the site, and they also automatically delete the post after six flags.
If there is a more serious situation that needs attention on the site and nobody from SE seems to be around, the easiest way to get some attention is to ping any moderator from any site you know in chat. Moderators can usually get hold of someone from SE reasonably quickly.
If it is just spam, another way is to link the spam in a high-traffic chat room you visit. That's often the fastest way to get enough spam votes to remove the posts.