Is there a way to tun off Z coordinates?

Why?  What are you doing wrong that some of your entities have a Z greater or lesser than zero?

to avoid having to flatten drawing and having new lines etc being drawn in 2d only without having a z coordinate and avoid having issues when filleting and trimming, etc.

You can force any Object Snapping to ignore Z coordinates in AutoCAD, with the OSNAPZ System Variable.  That won't change any already-existing things with non-zero Z coordinates.  But you can't "turn off" Z coordinates like those -- for some things you can collectively change Z coordinates to 0 in the Properties box, but for others FLATTEN may be the only choice if you really need them all in the XY plane.  I agree with the question about what is causing things to be non-zero in the Z direction.  If you're getting drawings from other that way, you can't control that, but if there are careless procedures in your operations, you should be able to improve on that.