John Gill Commentary Malachi 2:2

"If ye will not hear, and if ye will not lay it to heart, to give glory unto my name, saith Jehovah of hosts, then will I send the curse upon you, and I will curse your blessings; yea, I have cursed them already, because ye do not lay it to heart." — Malachi 2:2 (ASV)

If you will not hear
The commandment enjoined them; or the Gospel preached to them by Christ, and his apostles:

and if you will not lay [it] to heart to give glory to my name,
says the Lord of hosts ;
which they had despised and profaned before; if they did not take care of his worship and service, and honour the Messiah sent to them, in whom the name of the Lord was:

I will even send a curse upon you ;
both upon priests and people; those that bring the bad offerings, and those that receive them, as Kimchi; though Abarbinel restrains it to the priests:

and I will curse your blessings ,
either with which the priests blessed the people; or with which both they and the people were blessed; namely, their temporal blessings, such as their corn, and wine, and oil: and what wicked men have of this world, they have it with a curse, and not a blessing, as the righteous have; and therefore a little which they have, is better than much enjoyed by the wicked, (Psalms 37:16) :

yes, I have cursed them already ;
that is, from the time they began to despise his name, and not give him the glory due to him, as Kimchi and Abarbinel explain it:

because you do not lay [it] to heart ;
to glorify God.