Like many security policies, it merely has to be harder, not impossible.
There is an interesting decoding aspect where a "M" can never follow a "V" in a roman numeral representation. So its technically very easy to turn a written capital V into a written capital M but its not actually useful in practice.
Also there are word/letter spacing issues where technically one could remove "III" from the end of a numeral but it would probably result in typographically or graphic arts weird looking spacing.
There is an interesting decoding aspect where a "M" can never follow a "V" in a roman numeral representation. So its technically very easy to turn a written capital V into a written capital M but its not actually useful in practice.
Also there are word/letter spacing issues where technically one could remove "III" from the end of a numeral but it would probably result in typographically or graphic arts weird looking spacing.