I've found solution for my question. You can assign an action for each key inside xkb_symbols section. Redirect is an action we need:
key <AC07> { [ j, J, plusminus, NoSymbol, NoSymbol ], type[Group1]="EIGHT_LEVEL_SEMIALPHABETIC", actions = [ NoAction(), NoAction(), NoAction(), NoAction(), Redirect(key=<RTRN>, clearmods=all) ] };
Using this action you can clear existing modifiers or add new.