How to omit blessing the prophets in salah without making it invalid

To do that, simply replace any Tashahhud in the salah, be it after 2nd, 3rd or 4th rak´ah..., and include this one. You may also skip the Salat Ibrahim (Durood Sharif).

The new Tashahhud now looks like this:

Tashahhud (Sitting Position 2nd, 3rd or 4th... Rak'ah)

• After the last second Sujood in the rak´ah recite:

Attahiyyatu lillahi was-salawatu wat-tayyibaat
(All compliments, prayers, and pure words are due to Allah)

Assalamu 'alayna wa 'ala 'ibadillahis-saaliheen
(Peace be upon us and on the righteous servants of Allah)

Ashhadu alla ilaha illallah wa ashhadu anna Muhammadan 'abduhu wa rasuluh.
(I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and Messenger.)

Doing that you omit blessing the prophets if you think they do not deserve to be blessed.