If by "reject", you mean prevent that transaction from being mined and being available to your private key, then no.
If by "reject", you mean only act on rct payments, then you could ask the client for a monero address to return non-rct payment, possibly minus a small fee to cover the tx fee (for instance, I believe shapeshift will return your monero if you don't pay the exact amount, or if you send too late).
The intent of allowing both transaction types for a while is to leave some time for people to update, and not summarily reject transactions in too short a timeframe. Moreover, switching at once would mean the anonymity set for rct outputs would be pretty small for the first few rct outputs being sent in rct transactions, though that will fix itself quickly.
Please note that if you want rct outputs, you can resend any newly received non-rct outputs to yourself as an rct transaction.