In my application I use PIC16F676 microcontroler with the LM282 (https://www.onsemi.com/pub/Collateral/LM285-D.PDF) external voltage reference (for A/D converter) connected to the RA1 pin. This pin is also used during ICSP (Internal Circuit Serial Programming) as a ICSPCLK clock signal. How can I isolate this voltage reference from ICSPCLK programmer output and succesfully programing the chip?
I don't think there's a good way to do it. My first suggestion would be to just add a jumper in front of the LM282:
The jumper (or DIP-switch) should be closed during normal operations and open when programming.
If you absolutely cannot live with this requirement for manual intervention, you may throw in an op amp to avoid that node altogether:
This way your programmer will "fight" against the op-amp, but if the resistance (R2) is high enough, the programmer would "win". During normal operations, the op-amp just buffers the voltage reference. If you go that way, pick an op-amp that accepts inputs and outputs close to the rails - Rail-to-rail I/O.