If you've got the private key to the Bitcoin wallet the funds were sent to, you're the only one who can take 'em back out.  The person who sent the money in the first place wont have permission anymore once it leaves their wallet.  Your private key's like the key to your house - without it, no one can get inside your Bitcoin wallet except you.  So you gotta keep that private key locked down tight if you wanna make sure you're the only boss of your crypto stash.