It’s possible that you will receive many messages from WWOOFers. We ask that you reply to those messages as a sign of respect and in order to comply with the WWOOFing Charter. However, if you receive too many requests, you may send a standard email stating "Thank you xxxx for your interest. I currently don’t have any availabilities / I’m full for the time being." Even a generic response is appreciated.
Another way to solve this problem is to log into your account and hide your profile temporarily from viewing. For this, log in to your account > click on Host profile > then Edit profile > and finally on Hide my profile.