I creating a smart contract for manage loans and I am searching some way to make me sure when the borrower defaulted pay back the loan, seizing some percentage of ether from the guarantor's account.
Is this possible?
Thanks in advance.
As already mentioned, you can't just take Ether from the borrower's account.
If I understood correctly, you are thinking of a smart contract which gives our Ether loans. This should work pretty similarly to a normal bank loan except everything is automatic - the borrower needs to store some collateral.
What sort of collateral you accept is up to you. If the point is to borrow Ethers you probably shouldn't try to ask Ethers as collateral. Maybe some ERC20 tokens?
There are some similar services out there already, for example ethlend.