Why does Decius attempt to stop Artemidorus from giving Caesar his paper in Julius Caesar? Is he successful?

It does not seem certain that Decius is attempting to stop Artemidorus from handing Caesar his petition. In Act II, Scene 3, Artemidorus reads his letter to Caesar aloud to himself, and finishes the brief scene by saying:

If thou read this, O Caesar, thou mayst live.
If not, the fates with traitors do contrive.

There is no suggestion that anyone else knows about the contents of his letter. Therefore, Decius would have no reason for attempting to stop Artemidorus from handing it to Caesar, and cannot be said to be "successful" in preventing Artemidorus from getting Caesar to accept it. Decius just has another request and intrudes disrespectfully on Artemidorus, who does not seem to have the status of some others gathered around Caesar. When Artemidorus insists on having his letter accepted, Caesar says,

What, is the fellow mad?

And Publius says,

Sirrah, give place.

"Sirrah" is a contemptuous form of address. And when Caesar refers to Artemidorus as "the fellow" it suggests that he doesn't even know him. There are many men around Caesar who want something from him; Shakespeare cannot show them all. Caesar is trying to be modest when he says:

What touches us ourself shall be last served.

If Caesar had listened to Artemidorus, he might have lived. "The fates" were against it. Caesar was showing off his modesty and humility. Artemidorus did not have as much importance as many of the other men clustered around Caesar. Artemidorus is identified in the cast of characters as "a doctor of rhetoric." He is just another voice in the crowd, like the Soothsayer.

There may have been quite a few citizens who had heard about the conspiracy against Caesar. We see that one of the senators, Popillius Laena, also seems to know what the conspirators have planned. He approaches Cassius and says, in a stage whisper, 

I wish your enterprise today may thrive.

Cassius, characteristically, is suspicious of everybody. He knows he has been talking to all sorts of people about the danger Caesar presents to Rome—not necessarily recruiting them but sounding them out about their political sympathies. He could easily have made people suspicious, and there could have been a lot of rumors about a conspiracy. When Cassius says to Artemidorus,

What, urge you your petitions in the street?
Come to the Capitol,

he is showing that he would like to prevent all petitioners from getting to Caesar. Cassius is anxious to get Caesar to the Capitol, because that is where the assassination is to take place. It is noteworthy Cassius is also rude to Artemidorus, who lacks status among this gathering of all the most important men in Rome.

Both Artemidorus and Popillius may be intended to represent the outsiders who know something about the conspiracy. Popillius exemplifies the type of person who is solely concerned about himself, whereas Artemidorus exemplifies people who might try to interfere. Popillius is working both sides of the street, so to speak. If the assassination is successful, he would remind Cassius that he was on his side all along. If Caesar is killed, Popillius wants to be in good standing with the new king, which explains why he wishes Cassius good luck with his "enterprise" and then goes directly to Caesar and engages him in conversation. Cassius is nervous and suspicious, but Brutus tells him:

Cassius, be constant.
Popillius Laena speaks not of our purposes,
For look, he smiles, and Caesar does not change.

The audience has waiting through two acts while Shakespeare has been building up suspense. Here at the beginning of Act III, Scene 1, Shakespeare seems to be trying the audience feel as though the assassination will be unsuccessful after all.