Altostratus
NacreousHi, So the example I gave was for a monitor, so you would have to alter the /platform/mode.jsp (for example) on the server you wished to remove from the pool, so that it doesn't return "normal" any more.
I only answered part of your question however as you had previously mentioned iRules. You can momentarily mark a server down using LB::down, however the next monitor execution will bring it back up, and if you are running a version prior to 11.4 there is no supported way of bridging TMM (iRules) and the management plane (configuration), although you could have an external process that could monitor log messages generated by an iRule and then use iControl to remove members from a pool, but it's definitely a bespoke configuration that you would have to develop/support yourself.
If you have v11.4 or greater you can use iCall which is their newish all-singing/dancing feature that lets you alter configuration from an iRule. I haven't used it yet but here is some doco;-
https://devcentral.f5.com/articles/icall-all-new-event-based-automation-system.UtXwtEq4aM8
https://devcentral.f5.com/wiki/iCall.Mark_Server_Down_on_Excessive_Errors.ashx