Here is a page from a fairly recently published translation of the Tzetel Katan that has accompanying elucidations as well as prayers and exercises. While this doesn't point to a source per se for the Noam Elimelech's advice, it does shed some light on this method of clearing one's thoughts.
"You should immediately say, with all of your might: "HaCana'ani, HaChiti..."
Perhaps the reason for this is as the Bnei Yisaschar writes (Mamarei Chodshei Kislev-Teves, mamar 4, Hallel v'Hodaah, in the name of the Seer of Lublin), that the seven nations conquered by Yisroel are the seven sefiros of the Sitra Achra, the Side of Evil (corresponding to the seven sefiros of holiness), in the following order: HaCana'ani - Chesed; HaChiti - Gevurah; HaEmori - Tiferes; HaPrizi - Netzach; HaChivi - Hod; HaYivusi - Yesod; HaGirgoshi - Malchus.
WE CAN FURTHER EXPLAIN THIS, AS FOLLOWS:
Cana'ani is related to the word hachna'ah - "humility". However, this is a sort of meekness that originates from the forces of evil (kelipos). It is opposite the ideal state, in which a person's heart is "lifted up in the way of God" (Divrei HaYamim 17:6).
Chiti, from the word chet - "fear" (חת). This causes a person to constantly fear that his repentance is not effective.
Emori, from the word emor - "speak". This force does not stop speaking to a person for a minute, trying to subtly entice him and trap him in various types of sin.
Prizi, from the word "unwalled" (as in "unwalled cities" - ערי הפרזות). This relates to the trait of watonness; that is, when a person does not seek to be bound to his Creator, but imagines that he do whatever he desires.
Chivi, from the Aramaic word for "snake" (חיוא). It schemes like the primordial Serpent, first trying to seduce a person to sin. Then it bites him repeatedly [with negative feelings of guilt and remorse].
Yivusi, from the word "to trample" (as in Tehillim 60:14). It seeks to trample and annihilate a person from this world and the next.
Girgoshi, from the word girushin - "divorce." It constantly floods a person's mind with foreign thoughts in order to break his connection to God and prayer. In addition, when a person is in the middle of doing a mitzvah, it suddenly wants to kill him, and applies various schemes and evil thoughts to prevent him from completing his deed.
These are the forces of impurity that a person must be wary of. (Madanei Melech on Tzetel Katan, 5)