Hi Boris:)
FormName is not unique and must be qualified error gives :( your syntax
when i tried like below

lcSource01=[user01]
lcTarget01=[userforms]

	Delete user01;
		From user01;
		INNER Join (Select formName;
		FROM (m.lcSource01) cSrc1;
		LEFT Join (m.lcTarget01) cSrc2 On cSrc1.formName == cSrc2.formName;
		WHERE cSrc2.formName Is Null) cSrc3;
		ON user01.formName == cSrc3.formName