You can remove an attribute value by setting it to nothing (a longer explanation is below)
$MyAttr=;
Note the semicolon is not quoted, even for attribute type like strings. You can also use a empty string for string types:
$MyString="";
Only the first of the syntaxes above also allows attributes that use prototype or preference settings to re-inherit these again. See about resetting inheritance for more detail.
To explain the syntax further, you are effectively doing this to 'reset' an attribute to default inherited value:
$MyAttr=
and then explicitly then adding a semi-colon to close the expression. Thus the semi-colon isn't part of the process. So the last code above, if used as a rule on its own, will work but is not recommended. The additional semi-colon makes sure Tinderbox doesn't have to guess your intent and also avoids you having to later remember to add a semi-colon if you add more code on the end of the original unclosed expression.