Well Josh, it really depends on where that list of previously entered values is coming from. You will be able to tell by checking the Field/Control Setup dialog. Go into Layout mode (Ctrl-L or Cmd-L) and double-click the field.
One option is that the field is set to auto-complete using previously entered values.
Another option is that the field is set to display a value list defined to use values from the field.
Either way, the values you see reflect the index of the field. That is, you are seeing an index of all values entered into that field in that table. To clear the list, you would have to clear all data from that field in all records - obviously not what you want! To remove certain values from the list, you would need to find records with those values in that field and either delete the data or change it to something else.
The upshot is this - why do you want to "clear or manage" the list? Would it be easier to manage a manually entered (custom values) list? I am only guessing from here.