Is it permissible to say that some acts of worship are forbidden, such as prayer if it is done out of ostentation (riya'), and are there any fabricated hadiths?

Prayer performed out of ostentation (riya') has two scenarios: The first is that the sole motivation for performing it is ostentation. This is forbidden, invalid, and its perpetrator is sinful. The second is that the motivation is a combination of worshiping Allah and ostentation. This has three forms: when the aspect of sincerity predominates, or the aspect of ostentation predominates, or they are equal. In either of these cases, the perpetrator has committed something forbidden, which is the intention of ostentation.

As for fabricated (mawdu'a) hadiths, they are invented narrations that liars falsely attribute to the Messenger of Allah, peace and blessings be upon him.