When you have finished exchanging data with another application in a DDE conversation, you must terminate that conversation:
DDETerminate ChannelNum
Once you terminate a conversation, Access Basic generates an error if you use any of the DDE functions with the channel number for the terminated conversation.
When you close Microsoft Access, it automatically terminates all active DDE conversations. However, you may want to terminate all conversations without closing Microsoft Access. To do so, you can use several consecutive DDETerminate statements, a loop, or a shortcut Access Basic provides:
DDETerminateAll
As its name implies, the DDETerminateAll statement terminates all active DDE conversations.