The severity of hitting another Jew

Hitting a fellow Jew:
A. The prohibition, severity and penalty:[1]
It is [Biblically] forbidden for one to hit another Jew and if one hits him he transgresses a negative command.[2] This prohibition applies even to a light hit.[3]
Lifting hand to hit:[4] Furthermore, anyone who lifts a threatening hand against his friend, even if he does not hit him, is called a Rasha.[5]
Cherem: Whoever hits another Jew [without Halachic allowance[6]] is automatically entered into a Cherem of the previous generations.[7] One may not join him to a Minyan of ten Jews for any matter of Kedusha until a Beis Din [of three Kosher Jews, similar to Hataras Nedarim[8]] removes the Cherem from him, after he accepts their judgment.[9] [This applies even if the victim does not desire the Cherem to be removed, nevertheless, the Beis Din has authority to remove it.[10] Unfortunately, today there are many people who are in Cherem due to hitting and other reasons and it is not possible to enforce every Cherem, and that the person not join a Minyan. Nevertheless, a community should try as much as to enforce this matter.[11]] 
Lashes/payment:[12] If the hit caused a Peruta’s worth of damage to the person is liable to pay for the damages, and if it was a light hit that did not cause a Peruta’s worth of damage, he is liable for lashes.[13]
 
Summary:
One who hits another Jew transgresses a negative command, is considered a Rasha, and is liable for lashes or payment. He is in automatic Cherem and cannot join a Minyan until he accepts judgment from a quorum of three Kosher Jews and has them remove his Cherem. Even if one lifts his hand to hit a Jew he is considered a Rasha.
 

May one answer Amen to his blessings?[14]
Yes. A Jew who hits is in Cherem although is not considered like a gentile and hence one must answer Amen to his blessings.
 
What is one to do if a person who hit is Chazan?[15]
If a person who hit another Jew did not go to a Beis Din to receive judgment and have his Cherem removed, then if he forces himself as a Chazan upon a Minyan, the congregation is not to answer Amen to his blessings, and is to force him out of the position. If, however most of the congregation has accepted him and Chazan and the protest of the few individuals will not be useful to remove him from being Chazan, then they are to answer Amen.

May one hit someone as a joke?

No. Thus, one is not to slap someone’s back in a way that can cause him pain.