After setting a "Set Sequence Field" in a particular column in data grid, there is no way to remove or cancel it.
1. Close Toad
2. Go to your Toad Folder
3. Open the Toad.ini file
4. Use the Find feature to find "Sequences"
5. When you reach a result that looks like "[SEQUENCES]", delete the line under it (see below)
MDTEST.TKD_STUDENTS=STUDENT_ID^STUDENT_IDS <<<< delete this line
6. Save, Exit, and Start Toad