Convert MWBC to single 240V receptacle
Cap the neutral off and you’re done.
However, since your breaker is 2-pole with common trip, you’re perfectly allowed to eat your cake and have it too.  You can replace one side’s receptacle with either

a dual NEMA 6-15
a dual or single NEMA 6-20,
or a combo NEMA 5-20 NEMA 6-20, giving one socket of each.  Come off the hot of the other receptacle to the 2nd hot on the NEMA 6-20.

That last option lets you make full use of the MWBC, and then, make full use of the 240V circuit... although, not both at once at max amperage.  So an 8A/240V load will play nice with two 12A/120V loads, etc.