ExceptHeads is an option of FCLoopIsolate, FCDiracIsolate and other functions. It takes a list of heads that are not allowed to appear inside isolated expression.
For example, ExceptHeads -> {DiracGamma} in FCLoopIsolate blocks loop integrals where loop momenta are contracted with Dirac matrices.
Overview, FCLoopIsolate, FCDiracIsolate.