You sometimes have to ignore well intentioned bad advice.
Might see if there's a configuration / options that can be
adjusted prior to running ERC.
Or, make the clamp device sit under a symbol which has
D, G, S, B pins and looks like a MOSFET, but is not one
that gets recognized as such; unless ERC runs hierarchically,
it will not "see" that below is a naughtily-connected
MOSFET.
In Cadence you could insert a trivial presistor and then
the MOSFET will not be seen as directly connected.
In fact, since you're trying to deal with amps of current
traveling long distances, some effort to represent the
bussing resistance is a good idea (I'd bet your clamp
will need to size up, if you put a few thousand squares
of worst case metal resistance in series). That's why
clamps are often made small and distributed about the
periphery, even more than one per corner.
You should add an explicit back-diode, MOSFET taps
can be allowed way too sparse to be useful as B-D
diodes in forward conduction.
Also, this FET should follow ESD design rules (which
might improve the body diode some, but check).