If you have access to the machine within the SEPM and it's checking in you can allocate it to a group to remove the need for password.
Login to your SEPM click on clients > at the bottom click ad group <name group> click on policies tab > then passwords top right uncheck any password requirements and click ok.
Next click on top level group my computer and search for machine you are trying to uninstall from and move it to the newly created group witin the SEPM > once in the correct group log onto the machine right click on yellow SEP bottom right of task bar and select update policy wait about a minute and then try uninstalling. If done correctly then it won't ask for a password.
If you have no access to the SEPM then a support case will need to be raised to get access to cleanwipe or if you have 12.1 RU5 or above cleanwipe can be found in the tools part of the SEPM download from symantec.