I do not agree replies would automatically be spam.
I suggest you provide links to useful information. If you have a blog or web site that provides useful information that can also share that your services may be of value.
Certainly responding could be done in a spamming way. And that should not be done. But you can respond by being helpful. And rely on some of those seeing that you provide useful information wanting to learn more.
A measure of if you are providing useful replies see how often it is retweeted. I am very surprised how often mine are. I would guess it is over 50% of the time that I suggest a link would be useful they retweet it.
When I just send a reply without a link they are retweeted rarely. Also when I just post links to useful stuff I find those are retweeted much less often than my direct replies (I imagine if you have tens of thousands of active readers, not just "followers," this data would be less useful because everything you tweet someone retweets...
Another measure would be if people reply by saying you are sending them spam or wasting their time, etc. I have had 0 of these.
The combination of these results has led me to offer suggested links more often. I was nervous at first about people seeing it as spam. 90% of them are links to my blogs because those blog posts are what I know well enough to link so often (I also suggest other articles, blogs or products but less often).