To restrict the edit control to syntax memo, set the ReadOnly property to True. Set ReadOnly to False to allow the contents of the syntax memo to be edited. 

Setting ReadOnly to True ensures that the text is not altered, while still allowing the user to select text. The selected text can then be manipulated by the application, or copied to the Clipboard. 

To show caret in read only mode add soAlwaysShowCaret to Options, otherwise caret will be hidden if ReadOnly is True.

TCustomSyntaxMemo Properties