What exactly is highlighted?
Keywords and symbols are done by the highlighter. And if you set it to nil, then they should no longer highlight (with immediate effect).
Maybe try calling SynEdit.Invalidate; after removing the HL
Matching Bracket, or the highlight of the "word at caret" are done by various Markup modules.
SynEdit.MarkupByClass[TSynEditMarkupHighlightAllCaret].enabled := false;
SynEdit.MarkupByClass[TSynEditMarkupBracket].enabled := false;
SynEdit.MarkupByClass[TSynEditMarkupSpecialLine].enabled := false;
Similar for
TSynEditMarkupWordGroup
TSynEditMarkupCtrlMouseLink
TSynEditMarkupSpecialChar
If you do a copy of SynEdit it is just a few lines of code that need to be removed, to remove the entire markup.
Currently they are all added in the constructor.
If anything else highlights, maybe attach a screenshot.