Edit the pledge(1) you wish to stop/cancel and perform the following steps:
a) Set Perpetual? to NO if it is YES

b) Select "Select Payment Method" from the Payment Method dropdown
NOTE: The credit card must be removed from ALL pledges if the credit card is to be deleted from the Profile Payment Options tab.

c) Click Delete Outstanding Pledges

d) Save the pledge
(1) The pledge can be edited from,
Workspace > CRM > Constituents, the Contact Profile Pledges tab
Workspace > Fundraising > Pledges (click on an Amount)
Workspace > Fundraising > Gifts (filter for Types 'Pledge')