Rather than attempt to prevent some number of lines near the bottom of the Terminal window from being used, you might want to instead simply resize the Terminal window to whatever best serves your needs. You can put your mouse pointer over the lower-right corner of the Terminal window (or, really, over anywhere on the lower edge) until the mouse pointer changes to a different icon representing resizing. Then click and hold, and drag the border upwards until the window is however small you want it.
To make it so that all Terminals will be of a custom height (and, if you like, width) when you start them, go to
Profile Preferences in Terminal. A window with the title
Editing Profile "Default" will come up (or it will give a different profile name, if you're using a named custom profile). In the
General tab (which should already be showing), check the box labeled
Use custom default terminal size. Then you can decrease the number of
rows from 24 to whatever you like.
If your Terminal window is maximized, so that it takes up the whole screen and cannot be resized, then you should restore it to normal size (using the buttons on the window decorator, i.e., on the bar at the top of the window). Then you can either move it to a different location on the screen so that it is more convenient, or resize it as described above.