Thank you for your post.
I'm assuming you are talking about the "Email Campaign Management" sample template that ships with FileMaker Pro 11.
As I look through the different Layouts, I do see the "Record Detail Campaigns" Layout has a button to "Send" a campaign. This calls the script "Send Campaign". As I look through this script, I see a couple of Loops. The first loop just skiips between records and exits after the last record. This would not cause an infinite loop. However, the second loop evaluates the "g_Contact List_Temp" field (appears to be global storage) in the Campaigns table to a blank character. If this value is a space, then this would cause the loop to continue indefinitely. Since this global field is populated by the field "k_ID_Contact" in the Contacts table, make sure you don't have any entries with a space.
Let me know if you need additional clarification.