Some claim nowadays that ‘Scotch' should be used only in reference to whisky and perhaps certain foodstuffs such as Scotch Egg or in phrases like ‘Scotch mist'.
Regarding people, 'Scotch' was used as an adjective, regularly and quite happily, by Rabbie Burns as well as Sir Walter Scott, two of Scotland's major writers. The dislike some modern Scots have of the word in this context is really just based on a silly nationalistic affectation of the late 19th/early 20th centuries. There is no truly historical or etymological reason for ‘Scotch' to be considered demeaning or wrong at all.
However, nowadays it's just a harmless quirk with us Scots, so just humour us, but you're free to call ME a Scotchman if you like. If it was good enough for Rabbie, it's good enough for me!