unknown account under properties, security tab

there are 2 accounts that are stated as unknown accounts with a bunch of

numbers and letters following that under properties/security tab

whenever i right-click on any file in my computer. I think they got

there because i changed permissions of a account before i deleted it

months ago which was my mistake and ever since i created 1 more account

without touching any permissions but when i went to delete it, it

created a unknown account as well when i deleted it so now i have 2

unknown accounts, how can i remove these two unknown accounts if it is

at all possible

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hello2007