First You should find the index of that letter from companyinfos list the remove that index Try this and let me Know

for (Integer i = (companyinfos.size()-1) ; i>= 0 ; i--) { if(companyinfos.contains('a')){ Integer index =companyinfos.indexOf('a'); companyinfos.remove(index); } }

First You should find the index of that letter from companyinfos list the remove that index Try this and let me Know

for (Integer i = (companyinfos.size()-1) ; i>= 0 ; i--)
{
    if(companyinfos.contains('a')){
    Integer index =companyinfos.indexOf('a');
    companyinfos.remove(index);
    }
} 

             for (Integer i=(usedServiceAccList.size()-1);i>=0;i--){  
              String firstindex=usedServiceAccList[i];
                  if(ServAcc.contains(firstindex))
                  {
                    Integer index = copyServAcc.indexOf(firstindex);
                    copyServAcc.remove(index);
                    system.debug('removed--->'+copyServAcc);
                    system.debug('removed size--->'+copyServAcc.size()); 
                  }
               }

First You should find the index of that letter from companyinfos list the remove that index Try this and let me Know

for (Integer i = (companyinfos.size()-1) ; i>= 0 ; i--) { if(companyinfos.contains('a')){ Integer index =companyinfos.indexOf('a'); companyinfos.remove(index); } }