If you want to use the buyzone bitmask you should take a look at your zombie plugin. It probably provides a forward like "PlayerInfected" or something similar. You need to use that forward to also remove the buyzone bitflag if the player is already inside it.
The easiest way would be using CS_OnBuy from AMXX 1.8.3.
Alternatively you can use the plugin and API made by ConnorMcLeod:
https://forums.alliedmods.net/showthread.php?t=149380