Frankly, but looking at the code, there is no specific code for the shield.

So, my guess is that if you do not want a bot to buy a shield (considered as a primary weapon):
- specify a favorite weapon in the bot personality file, they should never drop their favorite weapon
- specify a very low priority (actually it is a probability as %) for the shield in buytable.ini
- move the shield down in the list in buytable.ini, first weapons have more probability to be selected than later ones (even with a lower priority)

NB: there may well be code missing to understand that a bot is carrying a shield. This could well be another problem.

Hope it helps

-eric