How can the user not uncheck what is available in enum list?

[BVBS]

Hi!

I'm building a document management app, but I'm having a problem with the forward action, I want users to be able to forward that text to other users through the [Nơi nhận] (Enum List) column but i don't want they can uncheck an existing one. So what should I do? Thank you for your help!

[AleksiAlkio]

Something like should work.. 
COUNT(INTERSECT([_THISROW_BEFORE].[EnumList],[EnumList]))=
COUNT([_THISROW_BEFORE].[EnumList])
and the field needs to be mandatory.