UnDeclareAllCommutators[] undeclares all user-defined commutators.
Overview, Commutator, CommutatorExplicit, DeclareNonCommutative, DotSimplify.
DeclareNonCommutative[a, b, c, d]
Commutator[a, b] = x1;
Commutator[c, d] = x2;
DotSimplify[a . b . c . d]b.a.d.c+\text{x2} b.a+\text{x1} d.c+\text{x1} \;\text{x2}
UnDeclareAllCommutators[]
DotSimplify[a . b . c . d]a.b.c.d