On your map's Settings tab, the Zoom section controls whether visitors can move around the map at all, how far they can zoom in, and which on-map controls show. This is also where you make a map fixed.
The master switch
Enable zoom is the master switch for zooming. Turn it off and the map is fixed at its start view: visitors can't zoom or pan, they just see the framed image. That suits a simple poster-style map. Leave it on for an explorable map.
How far, and how much slack
Maximum zoom level sets how far in visitors can go, as a multiple of the fit-to-screen size (for example 3 means three times in). Zoom margin (desktop) and Zoom margin (mobile / tablet) add pan slack past the map edges, so people can nudge the map a little beyond its border. A little breathing room helps on desktop, but on mobile and tablet it over-pans into empty space, so leave the mobile value at 0.
Which controls appear
Below those are toggles for the on-map controls:
Zoom + / − buttons. Show the zoom-in and zoom-out buttons on the map.
Clear button. Show a button that clears the current selection or directions.
Mouse-wheel zoom. Let the scroll wheel zoom the map (desktop only).
Fill the container. Keep the map image covering the whole frame at minimum zoom (desktop only; always on for mobile). See Control how your map fills the screen.
What visitors see
For how these controls feel from the visitor's side, see Zoom and pan a map.