Possibilities:
1)Use Coin Control to ignore it:
https://bitcointalk.org/index.php?topic=144331.0Send all other coins to somewhere else, delete the wallet.dat, open bitcoin-qt again and it will create a new wallet without those coins.
1b) without Coin Control: send the amount in your wallet -50 satoshis&fee somewhere else, delete the wallet.dat, open bitcoin-qt again and it will create a new wallet without those coins. (not sure if it will not select the 50 satoshis though, but imo it shouldn't)
2) make a raw transaction with those coins
3) Use Coin Control and mix it up with some transactions where it doesn't matter if every adress is known by someone else