You should first consider sending the tenant a letter or two concerning the vehicle. The more documented your efforts are concerning the issue, the better protected you are if the City gets involved. To the extent that the City issues citations, those citations will be attached to the property as @Ned Carey stated above. You might find yourself in a situation where you, as the landlord, will have to have the vehicles towed or take other measures to address citations issued to you by the City. Additionally, while you might not be able to pass this cost (or the cost of the violations, for example) as additional rent, that does not mean such costs wouldn't be considered damages.
There are reputable firms who can assist with the eviction process, but my recommendation would always be to use an attorney over an agent (in failure to pay rent cases). In a breach of lease action, you will have no choice but to use an attorney (or pursue it yourself, which is never advisable), especially if you believe that the tenant will appear and dispute. Baltimore City is tricky though, and if the tenant removes the vehicles prior to the hearing, some judges will be less inclined to give you possession of the property.
If this has become a pattern, you should also consider issuing a notice to terminate prior to the end of the tenancy. Speaking with an attorney will allow you to develop a sound plan to resolve these issues.