Do all signalling pathways have something that can inhibit them? If the signal pathway is benefitial and it is inhibited would the inhibitor be caused by a biological problem? Are all inhibitors controlled by some subcellular mechanism?
There are some inhibitors in the cell, which have specific effects on signal transduction. These are mostly second messengers (like cAMP, cGMP, calcium ions, inositol 1,4,5-trisphosphate and others), which are then able to diffuse relative fast to other proteins. There they can either amplify a signal or cause inhibiting processes to start which then terminate the activation of the signal transduction cascade. The problem is that these inhibitors that they work unspecific in their inhibition and termination. Signal transduction is tightly controlled as they directly influence the expression of various genes.
A lot of the proteins in these cascades are kinases which are activated upon phosphorylation and then carry on by phosphorylating the next step of the cascade. The activity is stopped by phosphatases which remove the phosphorylations from the kinases and thus inactivate them until the next signal is passed down the cascade. It works like in this figure (from here):
Specific inhibitors for single proteins in the signalling cascades are available as drugs against cancer. Since these signalling cascades regulate gene expression including cell cycle control, they are important targets for dysregulation in cancer. A numer of kinases in the MAP kinase pathway for example are mutated there and constantly active. The so called small molecule inhibitors bind to the protein and inhibit the kinase activity, preventing a permanent signal. See this article for more information: