in the kernel config file, you will see:

## The following flags may be set for the NCR53c94 based esp driver:
##      bits 0-7:  disable disconnect/reselect for the corresponding target
##      bits 8-15: disable synchronous negotiation for target [bit-8]

[ ... ]

dma0    at sbus0 slot ? offset ?                        # sun4c/sun4m
esp0    at sbus0 slot ? offset ? flags 0x0000           # sun4c


so, to disable both for all devices, use `flags 0xffff' there, and
rebuild your kernel.