The simplest way is to delete the data from the database. With that approach you don't need to find products, load and save them.
Multiselect attributes usually have the
backend_type = varchar. You can check that for your attribute by running the SQL statement:
select * from eav_attribute where attribute_code = '[YOUR_ATTRIBUTE_CODE]';
If it is a varchar attribute you can delete all values of this attribute for all products by running the following statement:
delete v from catalog_product_entity_varchar v
join catalog_product_entity p
on p.entity_id = v.entity_id
join eav_attribute a
on v.attribute_id = a.attribute_id and a.attribute_code = '[YOUR_ATTRIBUTE_CODE]';
If you want to test the statement for a single product first you can simply add
where p.sku = '[YOUR SKU] to the statement and it will delete only the values for that product.
I hope that helps.