This option enables you to specify a regexp with elements you want to exclude from parent mergeing. By default style information are attached to the parent element of a selection if all childrent/the whole element contents is selected. For example if you select a whole strong element and select a font size, the font-size style will be applied to the strong element and not with a wrapping style element. But sometimes you want to force a span element, this is where this option comes in to place. This option is set to nothing by default and applies to the font size/face/color and class drop drown lists.
Example of usage of the merge_styles_invalid_parents option:
tinyMCE.init({
...
// Excludes table cells and strong elements from any parent merge operation
merge_styles_invalid_parents : "^TD|STRONG$"
});