As explained in the help, set the 'MINPEAKHEIGHT' property to 5:
[peakValues, indexesOfPeaks] = findpeaks(smo, 'MINPEAKHEIGHT', 5);
or if you really want to set smo to zero below some threshold value, such as 7, you can do this first:
smo(smo<7) = 0;
[peakValues, indexesOfPeaks] = findpeaks(smo, 'MINPEAKHEIGHT', 5);
though I don't think that should be necessary with the data you've shown if you use the MinPeakHeight parameter.