Which is probably why it won't be much of a deterrent. They should require depositing a certain amount of BTC before you can send messages. On new accounts, or accounts with low stats, there could be a 3 day delay in withdrawing BTC, giving them time to identify people who are spamming. That way a spammer can't deposit BTC, send out a bunch of messages, and immediately withdraw the BTC. If someone is caught spamming, they just keep the BTC by banning the account, making it not worth it for the spammer. Accounts with enough stats, like 10 transactions and $500 in purchases, could send messages without the BTC requirement.