DISCLAIMER, this is all server sided, so the host must change this. Also, make sure you have a method of editing text files before starting.
First of all, go to colony survival in your library, and hit the gear above your achievements/friendslist.
It'll give you 4 options, hit the one that says "manage", then "browse local files".
That will open a file tab, with multiple files and applications.You will want to click the file that says "gamedata", then click "settings".
If done correctly, it will bring you to a file with 4 text documents saying "client", "permissiongroups", "server", and "serverperclient". The important file here is server, click into that.This will open the text file, and will give you access to changing a lot of things, like chunks that the banner loads, zombie spawn radius, etcetera. Underneath banner, there is a setting called "MonsterPathingRadius". I personally set this to 800, ,but a smaller tweak will grant generally the same effect if you are just doing smaller scale projects
And Tada! You can now increase the distance the zombies will travel before deciding they ain't got the time for that. Its technically more strenuous on your PC when you do this, as it is a larger radius the pathfinding has to detect for, but it will allow much more creative liberty before you get struck with "SIEGE MODE"