is OSM secure? Could I delete a whole town?

Please see that other question about OSM's data-verification-process. To answer your question in short: If "someone crazy registers and will erase everything" we can revert/undo (a task for rather experienced users) those changes. Also there are many backups of our database.

I do not know if there is, in addition, some server-side block if someone would attempt to delete / edit more than for example 100000 objects - could be (please do not try it).

The short answer is: No, OSM is not secure. Then, you might get in a philosophical discussion on what security means in this context. Do you mean "Can someone hack the database and permanently rename every town to PENIS?" - I'd say no. Just look at Wikipedia, where it is not possible to permanently do much damage either. All open community projects have to deal with vandalism, but the problem is not very large and some small measures (like having to create an account to be able to edit) restrict it very much.

Then, if you mean "Can I blindly trust what is on the map? Can I depend on the map for live-or-death-decisions?", the answer is also no, but this holds for any map. Say you are chased down a street by a couple of hungry utahraptors/velociraptors, and chose OSM as the means to not land in a dead end (quite literally ;) ), then depending on the area you might find OSM very incomplete or very complete. And you will leave the raptors either very hungry or very well fed.