Good day
I have since a few days the problem that new players appear on my server and get into the houses without breaking the walls or doors. Apparently with Noclip now I wanted to ask how I can fix something like that? have also installed an anti-cheat but do not understand how they do that and how I can prevent it.

This is not possible unless:

1. Players somehow get admin flag (status) or have somehow access to noclip command from the server via plugin that default group have permission for it.

2. If you carefully check 1. they do not have anything from it, then either kind of glitch is happening or cheating...

2.1 Not sure if glitches still exist but they can probably do something, chances are 0.0000001% so I think its...

2.2 Hacking... They probably hack and as I know cheats have ability to debug camera or even go through walls when watched video before years... You have walls invisible for you and you go through them like admin disable layers from Tools tab in F1. If they just noclip or fly/speedhack as normal players without admin status they should end up kicked by the server for FlyHack Violation etc so its weird stuff...

Also make sure bases are not of a newbs with twig parts and they complain later about sus activity when for example their roof is twig... 

Btw what anti-cheat you use?

I hope all this helps you somehow to fix your issue...

as above or do you have  this mod installed ?

https://umod.org/plugins/chute

players may use an exploit within this mod the enter a base.