how do I avoid defining words with names that conflict with built-in words?

It depends on what you want the Forth system do when you try to redefine a word. And should this apply to cases where you are redefining your own word.

Forth doesn’t actually distinguish between user-defined words and built-in words (including standard words).

Typically, Forth systems give a warning when you redefine a word in the same word list. Otherwise, even if the new definition shadows a word in another word list in the search order (or, conversely, a word in another word list in the search order shadows the new definition) — you do not get any warning.

A simplest way to prevent redefinitions of the latter kind is to redefine the defining words to check the corresponding condition.

An example for the word “:” (colon):

: : ( "name" -- colon-sys )
  >in @ >r  save the position in the input buffer
    parse-name find-name ( nt | 0 )
    abort" A word with the given name is available in the search order"
  r> >in !  restore the position in the input buffer
  :  call to the original ":"
;

Once you enter this definition, you will not be able to create a new definition for a name using : (colon) if a word with the same name is available in the search order. But you will still be able to redefine a word in a word list that is absent in the search order.

Also, you will be able to redefine a word using other defining words like defer, create, variable, etc. All of them can also be redefined in the way as to prevent further redefinitions of the mentioned kind. Of course, you should do it before redefining : (colon).

See also: How can I access a shadowed definition in Forth?