Is it possible to disable NAT selectively

you can be more selective than just the event itself, limiting to particular subnets or host. Details on snat behavior from iRules from the snat command page in the iRules wiki:

Causes the system to assign the specified source address to the serverside connection(s). The assignment is valid for the duration of the clientside connection or until 'snat none' is called. The iRule SNAT command overrides the SNAT configuration of the virtual server or a SNAT pool. It does not override the 'Allow SNAT' setting of a pool.