If you want to make available your OS deployment task sequences only via PXE :
For Configuration Manager 2007/2012 you can specify a client platform that does not exist in your environment in task sequence properties.
For Instance if you select Windows Vista SP2(64Bit), this TS will only be available on this platform PC’s. However, Windows PE(network boot) ignores platform restrictions, therefore this TS will be available for every collection/computer in PXE environment.
Also with the upcoming ConfigMgr 2012 Service Pack 1, you can control this settings via configuration on the deployment itself. You will get multiple options:
SP1 still in Beta and will be available soon. Till then, you can use first method.